PHP: Tutorial - Page 2

Constants

Constants are often used in functions that require specific values to be passed in. Use the define function to create constants.

<?php
	define("MAX", 1000);
	define("TRUE_MAX", MAX);
	define("USERNAME", "John");

	echo MAX;
	echo TRUE_MAX;
	echo USERNAME;
?>

References

A reference is a variable that points to another variable. Changing one changes both (or all that are referenced).

<?php
	$i = 321;
	$j = &$i; // $j is a reference to $i

	echo $i;
	echo $j;

	$i = 654;

	echo $i;
	echo $j;

	$j = 987;

	echo $i;
	echo $j;
?>

Operators

There are operators in PHP to handle arithmetic, assignment, comparison, etc.

operator	operation
=	assigns right operand to the left operand
==	is true when the two operands are equal
!=	is true when the two operands are not equal
===	is true when the two operands are identical
!==	is true when the two operands are not identical
<	is true when left operand is less than right operand
>	is true when left operand is greater than right operand
<=	is true when left operand is less than or equal to right operand
>=	is true when left operand is greater than or equal to right operand
&&	is true when both left and right operands are true
||	is true when either left or right operand is true
++	increment operand by one
--	decrement operand by one
+=	increments left operand by right operand
-=	decrements left operand by right operand
.	concatenates left and right operand
%	divides the left operand by the right and returns the remainder
|	bitwise OR: returns the bits that are on in either the left or right operands
&	bitwise AND: returns the bits that are one both in the left and right operands
?:	ternary operator is like a condensed conditional statement
`	execution operator (the backtick key)

An example using the execution operator:

<?php
	$output = `ls -al`;
	echo '<pre>';
	echo $output;
	echo '</pre>';
?>

Conditional Statement

A conditional statement takes a different action depending on the outcome of a test. Any number that is not zero is considered true so zero is consider false. Empty strings evaluate to false. The === operator compares values and data types, if they are the same data type with the same data then they are identical.

<?php
	$i = 10;

	if ($i) {
		echo '$i is not 0';
	}

	if(10 == "10") { // this is true
		echo "EQUAL";
	}

	if(10 === "10") { // this is not true
		echo "IDENTICAL";
	}

	if ($i == 0) {
		echo 'zero';
	}
	else if ($i < 5) {
		echo 'five';
	}
	else if ($i >= 100 && $i < 200) {
		echo 'huge';
	}
	else if ($i <= 10) {
		echo 'ten';
	}
?>

Switch Statement

Sometimes a switch is more appropriate for executing code based on conditions.

<?php
	$animal = 'dog';

	switch($animal) {
		case 'cat':
			echo 0;
			break;
		case 'pony':
			echo 1;
			break;
		case 'snake':
			echo 2;
			break;
		case 'dog':
			echo 3;
			break;
		default:
			echo 10;
	}
?>

Loop Statements

There are four ways to execute a block of code multiple times. These are with the while, for, foreach, and do while loop statements.

The while loop executes a block of code for as long as a given condition is true.

<?php
	$i = 0;
	while($i < 10) {
		echo $i;
		$i = $i + 1;
	}
?>

The for loop specifies a declaration, a condition, and an action.

<?php
	for($i = 0; $i < 10; $i = $i + 1) {
		echo $i;
	}
?>

The foreach loop is specifically for objects and arrays.

<?php
	$arr = array(1, 2, 3, 4, 5);
	foreach($arr as $i) {
		echo $i;
	}
?>

The foreach loop can access the keys and values of the array.

<?php
	$arr = array(5, 4, 3, 2, 1);
	foreach($arr as $key => $value) {
		echo "[$key] is set to $value\n";
	}
?>

The do while loop is like the while one except the condition is evaluated at the end of the code block.

<?php
	$i = 0;
	do {
		echo $i;
		$i = $i + 1;
	} while($i < 10);
?>

exit

Use the exit construct to terminate the processing of the PHP script file.

<?php
	exit;
	echo 'THIS WILL NOT BE OUTPUT!';
?>

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