Control Structures

if

The if construct is one of the most important features of many languages, PHP included. It allows for conditional execution of code fragments. PHP features an if structure that is similar to that of C:

if (expr) statement

The following example would display a is bigger than b if $a is bigger than $b:

<?phpif ($a > $b)     echo "a is bigger than b";?>

else

Often you'd want to execute a statement if a certain condition is met, and a different statement if the condition is not met. This is what else is for. else extends an if statement to execute a statement in case the expression in the if statement evaluates to FALSE. For example, the following code would display a is bigger than b if $a is bigger than $b, and a is NOT bigger than b otherwise:

<?phpif ($a > $b) {     echo "a is bigger than b"; } else {     echo "a is NOT bigger than b"; }?>

elseif

elseif, as its name suggests, is a combination of if and else. Like else, it extends an if statement to execute a different statement in case the original if expression evaluates to FALSE. However, unlike else, it will execute that alternative expression only if the elseif conditional expression evaluates to TRUE. For example, the following code would display a is bigger than b, a equal to b or a is smaller than b:

<?phpif ($a > $b) {     echo "a is bigger than b"; } elseif ($a == $b) {     echo "a is equal to b"; } else {     echo "a is smaller than b"; }?>

There may be several elseifs within the same if statement. The first elseif expression (if any) that evaluates to TRUE would be executed. In PHP, you can also write 'else if' (in two words) and the behavior would be identical to the one of 'elseif' (in a single word). The syntactic meaning is slightly different (if you're familiar with C, this is the same behavior) but the bottom line is that both would result in exactly the same behavior.
The elseif statement is only executed if the preceding if expression and any preceding elseif expressions evaluated to FALSE, and the current elseif expression evaluated to TRUE.

while

while loops are the simplest type of loop in PHP. They behave just like their C counterparts. The basic form of a while statement is:

while (expr) statement

The meaning of a while statement is simple. It tells PHP to execute the nested statement(s) repeatedly, as long as the while expression evaluates to TRUE. The value of the expression is checked each time at the beginning of the loop, so even if this value changes during the execution of the nested statement(s), execution will not stop until the end of the iteration (each time PHP runs the statements in the loop is one iteration). Sometimes, if the while expression evaluates to FALSE from the very beginning, the nested statement(s) won't even be run once.
Like with the if statement, you can group multiple statements within the same while loop by surrounding a group of statements with curly braces, or by using the alternate syntax:

while (expr): statement ... endwhile;

The following examples are identical, and both print numbers from 1 to 10:

<?php/* example 1 */$i = 1; while ($i <= 10) {     echo $i++;  /* the printed value would be                     $i before the increment                     (post-increment) */}/* example 2 */$i = 1; while ($i <= 10):     echo $i;     $i++; endwhile;?>

do-while

do-while loops are very similar to while loops, except the truth expression is checked at the end of each iteration instead of in the beginning. The main difference from regular while loops is that the first iteration of a do-while loop is guaranteed to run (the truth expression is only checked at the end of the iteration), whereas it's may not necessarily run with a regular while loop (the truth expression is checked at the beginning of each iteration, if it evaluates to FALSE right from the beginning, the loop execution would end immediately).
There is just one syntax for do-while loops:

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

The above loop would run one time exactly, since after the first iteration, when truth expression is checked, it evaluates to FALSE ($i is not bigger than 0) and the loop execution ends.
Advanced C users may be familiar with a different usage of the do-while loop, to allow stopping execution in the middle of code blocks, by encapsulating them with do-while (0), and using the break statement. The following code fragment demonstrates this:

<?phpdo {     if ($i < 5) {         echo "i is not big enough";         break;     }     $i *= $factor;     if ($i < $minimum_limit) {         break;     }    echo "i is ok";     /* process i */} while (0);?>

for

for loops are the most complex loops in PHP. They behave like their C counterparts. The syntax of a for loop is:

for (expr1; expr2; expr3) statement

The first expression (expr1) is evaluated (executed) once unconditionally at the beginning of the loop.
In the beginning of each iteration, expr2 is evaluated. If it evaluates to TRUE, the loop continues and the nested statement(s) are executed. If it evaluates to FALSE, the execution of the loop ends.
At the end of each iteration, expr3 is evaluated (executed).
Each of the expressions can be empty. expr2 being empty means the loop should be run indefinitely (PHP implicitly considers it as TRUE, like C). This may not be as useless as you might think, since often you'd want to end the loop using a conditional break statement instead of using the for truth expression.
Consider the following examples. All of them display numbers from 1 to 10:

<?php/* example 1 */for ($i = 1; $i <= 10; $i++) {     echo $i; }/* example 2 */for ($i = 1; ; $i++) {     if ($i > 10) {         break;     }     echo $i; }/* example 3 */$i = 1; for (; ; ) {     if ($i > 10) {         break;     }     echo $i;     $i++; }/* example 4 */for ($i = 1; $i <= 10; print $i, $i++);?>