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Using Operators

Other Operators

In addition to the operators we have covered so far,you can use several others.

The comma operator (

) separates function arguments and other lists of items.It is

normally used incidentally.

Two special operators,

new

and

,are used to instantiate a class and access class

members,respectively.They are covered in detail in Chapter 6.

There are a few others that we discuss briefly here.

The Ternary Operator

The ternary operator (

) works the same way as it does in C.It takes the following

form:

condition ? value if true : value if false

This operator is similar to the expression version of an

if-else

statement, which is cov-

ered later in this chapter.

A simple example is

($grade >= 50 ? ‘Passed’ : ‘Failed’)

This expression evaluates student grades to

‘Passed’

‘Failed’

The Error Suppression Operator

The error suppression operator (

) can be used in front of any expression—that is,any-

thing that generates or has a value. For example,

$a = @(57/0);

Without the

operator,this line generates a divide-by-zero warning.With the operator

included,the error is suppressed.

If you are suppressing warnings in this way,you should write some error handling

code to check when a warning has occurred.If you have PHP set up with the

track_errors

feature enabled,the error message will be stored in the global variable

$php_errormsg

The Execution Operator

The execution operator is really a pair of operators—a pair of backticks (

) in fact.The

backtick is not a single quotation mark;it is usually located on the same key as the

(tilde) symbol on your keyboard.

PHP attempts to execute whatever is contained between the backticks as a command

at the server’s command line.The value of the expression is the output of the command.

For example,under Unix-like operating systems,you can use

$out = `ls -la`;

echo ‘<pre>’.$out.’</pre>’;

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Chapter 1 PHP Crash Course

Or,equivalently on a Windows server,you can use

$out = `dir c:`;

echo ‘<pre>’.$out.’</pre>’;

Either version obtains a directory listing and stores it in

$out

.It can then be echoed to

the browser or dealt with in any other way.

There are other ways of executing commands on the server.We cover them in

Chapter 18,“Interacting with the File System and the Server.”

Array Operators

There are a number of array operators.The array element operators (

[]

) enable you to

access array elements.You can also use the

operator in some array contexts.These

operators are covered in Chapter 3.

You also have access to a number of other array operators.We cover them in detail in

Chapter 3 as well,but we included them here for completeness.

Table 1.6 PHP’s Array Operators

Operator

Name

Use

Result

Union

$a + $b

Returns an array containing everything in

and

Equality

$a == $b

Returns

true

and

have the same

elements

===

Identity

$a === $b

Returns

true

and

have the same

elements in the same order

Inequality

$a != $b

Returns

true

and

are not equal

Inequality

$a <> $b

Returns

true

and

are not equal

!==

Non-identity

$a !== $b

Returns

true

and

are not identical

You will notice that the array operators in Table 1.6 all have equivalent operators that

work on scalar variables.As long as you remember that

performs addition on scalar

types and union on arrays—even if you have no interest in the set arithmetic behind that

behavior—the behaviors should make sense.You cannot usefully compare arrays to scalar

types.

The Type Operator

There is one type operator:

instanceof

.This operator is used in object-oriented pro-

gramming, but we mention it here for completeness.(Object-oriented programming is

covered in Chapter 6.)

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Using Operators: Working Out the Form Totals

The

instanceof

operator allows you to check whether an object is an instance of a

particular class,as in this example:

class sampleClass{};

$myObject = new sampleClass();

if ($myObject instanceof sampleClass)

echo “myObject is an instance of sampleClass”;

Using Operators: Working Out the Form Totals

Now that you know how to use PHP’s operators,you are ready to work out the totals

and tax on Bob’s order form.To do this,add the following code to the bottom of your

PHP script:

$totalqty = 0;

$totalqty = $tireqty + $oilqty + $sparkqty;

echo ‘Items ordered: ‘.$totalqty.’ ’;

$totalamount = 0.00;

define(‘TIREPRICE’, 100);

define(‘OILPRICE’, 10);

define(‘SPARKPRICE’, 4);

$totalamount = $tireqty * TIREPRICE

+ $oilqty * OILPRICE

+ $sparkqty * SPARKPRICE;

echo ‘Subtotal: $’.number_format($totalamount,2).’ ’;

$taxrate = 0.10; // local sales tax is 10%

$totalamount = $totalamount * (1 + $taxrate);

echo ‘Total including tax: $’.number_format($totalamount,2).’ ’;

If you refresh the page in your browser window, you should see output similar to

Figure 1.5.

As you can see, this piece of code uses several operators. It uses the addition (

) and

multiplication (

) operators to work out the amounts and the string concatenation oper-

ator (

) to set up the output to the browser.

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Chapter 1 PHP Crash Course

Figure 1.5 The totals of the customer’s order have been calculated,

formatted, and displayed.

It also uses the

number_format()

function to format the totals as strings with two

decimal places.This is a function from PHP’s Math library.

If you look closely at the calculations,you might ask why the calculations were per-

formed in the order they were.For example,consider this statement:

$totalamount = $tireqty * TIREPRICE

+ $oilqty * OILPRICE

+ $sparkqty * SPARKPRICE;

The total amount seems to be correct,but why were the multiplications performed

before the additions? The answer lies in the precedence of the operators—that is, the

order in which they are evaluated.

Understanding Precedence and Associativity:

Evaluating Expressions

In general,operators have a set precedence,or order, in which they are evaluated.

Operators also have an associativity,which is the order in which operators of the same

precedence are evaluated.This order is generally left to right (called left for short),right

to left (called right for short), or not relevant.

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Understanding Precedence and Associativity: Evaluating Expressions

Table 1.7 shows operator precedence and associativity in PHP.In this table, operators

with the lowest precedence are at the top, and precedence increases as you go down the

table.

Table 1.7 Operator Precedence in PHP

Associativity

Operators

left

xor

left

and

right

left

= += -= *= /= .= %= &= |= ^= ~= <<= >>=

left

? :

left

n/a

== != === !==

n/a

< <= > >=

left

<< >>

left

+ - .

left

* / %

right

! ~ ++ -- (int) (double) (string) (array) (object) @

right

[]

n/a

new

n/a

()

Notice that we haven’t yet covered the operator with the highest precedence: plain old

parentheses.The effect of using parentheses is to raise the precedence of whatever is con-

tained within them.This is how you can deliberately manipulate or work around the

precedence rules when you need to.

Remember this part of the preceding example:

$totalamount = $totalamount * (1 + $taxrate);

If you had written

$totalamount = $totalamount * 1 + $taxrate;

the multiplication operation,having higher precedence than the addition operation,

would be performed first, giving an incorrect result.By using the parentheses,you can

force the subexpression

1 + $taxrate

to be evaluated first.

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Chapter 1 PHP Crash Course

You can use as many sets of parentheses as you like in an expression.The innermost

set of parentheses is evaluated first.

Also note one other operator in this table we have not yet covered:the

lan-

guage construct,which is equivalent to

echo

.Both constructs generate output.

We generally use

echo

in this book,but you can use

if you find it more read-

able.Neither

nor

echo

is really a function,but both can be called as a function

with parameters in parentheses.Both can also be treated as an operator:You simply place

the string to work with after the keyword

echo

Calling

as a function causes it to return a value (

).This capability might be

useful if you want to generate output inside a more complex expression but does mean

that

is marginally slower than

echo

Using Variable Functions

Before we leave the world of variables and operators,let’s look at PHP’s variable func-

tions.PHP provides a library of functions that enable you to manipulate and test vari-

ables in different ways.

Testing and Setting Variable Types

Most of the variable functions are related to testing the type of function.The two most

general are

gettype()

and

settype()

.They have the following function prototypes;that

is,this is what arguments expect and what they return:

string gettype(mixed var);

int settype(mixed var, string type);

To use

gettype()

,you pass it a variable.It determines the type and returns a string con-

taining the type name:

boolean

integer

double

(for floats),

string

array

object

resource

,or

NULL

.It returns

unknown type

if it is not one of the standard types.

To use

settype()

,you pass it a variable for which you want to change the type and a

string containing the new type for that variable from the previous list.

Note

This book and the php.net documentation refer to the data type mixed. There is no such data type, but

because PHP is so flexible with type handling, many functions can take many (or any) data types as an

argument. Arguments for which many types are permitted are shown with the pseudo-type mixed.

You can use these functions as follows:

$a = 56;

echo gettype($a).’ ’;

settype($a, ‘double’);

echo gettype($a).’ ’;

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Using Variable Functions

When

gettype()

is called the first time,the type of

is integer.After the call to

settype()

,the type is changed to

double

PHP also provides some specific type-testing functions.Each takes a variable as an

argument and returns either

true

false

.The functions are

is_array()

is_double()

is_float()

is_real()

(All the same function)

is_long()

is_int()

is_integer()

(All the same function)

is_string()

is_object()

is_resource()

is_null()

is_scalar()

—Checks whether the variable is a scalar,that is, an integer,boolean,

string,or float.

is_numeric()

—Checks whether the variable is any kind of number or a numeric

string.

is_callable()

—Checks whether the variable is the name of a valid function.

Testing Variable Status

PHP has several functions for testing the status of a variable.The first is

isset()

,which

has the following prototype:

boolean isset(mixed var);

This function takes a variable name as an argument and returns

true

if it exists and

false

otherwise.You can also pass in a comma-separated list of variables,and

isset()

will return

true

if all the variables are set.

You can wipe a variable out of existence by using its companion function,

unset()

which has the following prototype:

void unset(mixed var);

This function gets rid of the variable it is passed.

The

empty()

function checks to see whether a variable exists and has a nonempty,

nonzero value;it returns

true

false

accordingly. It has the following prototype:

boolean empty(mixed var);

Let’s look at an example using these three functions.

Try adding the following code to your script temporarily:

echo ‘isset($tireqty): ‘.isset($tireqty).’ ’;

echo ‘isset($nothere): ‘.isset($nothere).’ ’;

echo ‘empty($tireqty): ‘.empty($tireqty).’ ’;

echo ‘empty($nothere): ‘.empty($nothere).’ ’;

Chapter 1 PHP Crash Course

Refresh the page to see the results.

The variable

$tireqty

should return 1 (

true

) from

isset()

regardless of what value

you entered in that form field and regardless of whether you entered a value at all.

Whether it is

empty()

depends on what you entered in it.

The variable

$nothere

does not exist,so it generates a blank (

false

) result from

isset()

and a 1 (

true)

result from

empty()

These functions can be handy when you need to make sure that the user filled out

the appropriate fields in the form.

Reinterpreting Variables

You can achieve the equivalent of casting a variable by calling a function.The following

three functions can be useful for this task:

int intval(mixed var[, int base]);

float floatval(mixed var);

string strval(mixed var);

Each accepts a variable as input and returns the variable’s value converted to the appro-

priate type.The

intval()

function also allows you to specify the base for conversion

when the variable to be converted is a string. (This way,you can convert, for example,

hexadecimal strings to integers.)

Implementing Control Structures

Control structures are the structures within a language that allow you to control the flow

of execution through a program or script.You can group them into conditionals (or

branching) structures and repetition structures (or loops).We consider the specific imple-

mentations of each of them in PHP next.

Making Decisions with Conditionals

If you want to sensibly respond to your users’ input, your code needs to be able to make

decisions.The constructs that tell your program to make decisions are called conditionals.

if Statements

You can use an

statement to make a decision.You should give the

statement a

condition to use.If the condition is

true

,the following block of code will be executed.

Conditions in

statements must be surrounded by parentheses

()

For example,if a visitor orders no tires, no bottles of oil,and no spark plugs from

Bob,it is probably because she accidentally clicked the Submit Order button before she

had finished filling out the form. Rather than telling the visitor “Order processed,”the

page could give her a more useful message.

Making Decisions with Conditionals

When the visitor orders no items,you might like to say,“You did not order anything

on the previous page!”You can do this easily by using the following

statement:

if( $totalqty == 0 )

echo ‘You did not order anything on the previous page! ’;

The condition you are using here is

$totalqty == 0

.Remember that the equals opera-

tor (

) behaves differently from the assignment operator (

The condition

$totalqty == 0

will be

true

$totalqty

is equal to zero.If

$totalqty

is not equal to zero,the condition will be

false

.When the condition is

true

,the

echo

statement will be executed.

Code Blocks

Often you may have more than one statement you want executed according to the

actions of a conditional statement such as

.You can group a number of statements

together as a block.To declare a block,you enclose it in curly braces:

if( $totalqty == 0 )

{

echo ‘’;

echo ‘You did not order anything on the previous page! ’;

echo ‘’;

}

The three lines enclosed in curly braces are now a block of code.When the condition is

true

,all three lines are executed.When the condition is

false

,all three lines are

ignored.

Note

As already mentioned, PHP does not care how you lay out your code. However, you should indent your code

for readability purposes. Indenting is used to enable you to see at a glance which lines will be executed only

if conditions are met, which statements are grouped into blocks, and which statements are parts of loops or

functions. In the previous examples, you can see that the statement depending on the ifstatement and

the statements making up the block are indented.

else Statements

You may often need to decide not only whether you want an action performed,but also

which of a set of possible actions you want performed.

else

statement allows you to define an alternative action to be taken when the

condition in an

statement is

false

.Say you want to warn Bob’s customers when they

do not order anything.On the other hand,if they do make an order, instead of a warn-

ing,you want to show them what they ordered.

Chapter 1 PHP Crash Course

If you rearrange the code and add an

else

statement,you can display either a warn-

ing or a summary:

if( $totalqty == 0 )

{

echo ‘You did not order anything on the previous page! ’;

}

else

{

echo $tireqty.’ tires ’;

echo $oilqty.’ bottles of oil ’;

echo $sparkqty.’ spark plugs ’;

}

You can build more complicated logical processes by nesting

statements within each

other.In the following code,the summary will be displayed only if the condition

$totalqty == 0

true

,and each line in the summary will be displayed only if its own

condition is met:

if( $totalqty == 0)

{

echo ‘You did not order anything on the previous page! ’;

}

else

{

if ( $tireqty>0 )

echo $tireqty.’ tires ’;

if ( $oilqty>0 )

echo $oilqty.’ bottles of oil ’;

if ( $sparkqty>0 )

echo $sparkqty.’ spark plugs ’;

}

elseif Statements

For many of the decisions you make,you have more than two options.You can create a

sequence of many options using the

elseif

statement,which is a combination of an

else

and an

statement.When you provide a sequence of conditions,the program can

check each until it finds one that is true.

Bob provides a discount for large orders of tires.The discount scheme works like this:

Fewer than 10 tires purchased—No discount

10–49 tires purchased—5% discount

50–99 tires purchased—10% discount

100 or more tires purchased—15% discount

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