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Control Structures

while
do while
for
foreach
break
continue

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

While loops 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;

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.

In do-while, the first iteration of a do-while loop is guaranteed to run (the truth expression is only checked at the end of the iteration).

There is just one syntax for do-while loops:

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

PHP also supports the alternate "colon syntax" for for loops.

for (expr1; expr2; expr3):
statement
...
endfor;.

 

foreach


foreach simply gives an easy way to iterate over arrays. foreach works only on arrays, and will issue an error when you try to use it on a variable with a different data type or an uninitialized variable. There are two syntaxes; the second is a minor but useful extension of the first:

foreach (array_expression as $value)
statement
foreach (array_expression as $key => $value)
statement

The first form loops over the array given by array_expression. On each loop, the value of the current element is assigned to $value and the internal array pointer is advanced by one (so on the next loop, you'll be looking at the next element).

The second form does the same thing, except that the current element's key will be assigned to the variable $key on each loop.

break


break ends execution of the current for, foreach, while, do-while or switch structure.

break accepts an optional numeric argument which tells it how many nested enclosing structures are to be broken out of.

<?php
$arr = array('one', 'two', 'three', 'four', 'stop', 'five');
while (list(, $val) = each($arr)) {
if ($val == 'stop') {
break; /* You could also write 'break 1;' here. */
}
echo "$val<br />\n";
}

/* Using the optional argument. */

$i = 0;
while (++$i) {
switch ($i) {
case 5:
echo "At 5<br />\n";
break 1; /* Exit only the switch. */
case 10:
echo "At 10; quitting<br />\n";
break 2; /* Exit the switch and the while. */
default:
break;
}
}
?>

continue


continue is used within looping structures to skip the rest of the current loop iteration and continue execution at the condition evaluation and then the beginning of the next iteration.

Note: Note that in PHP the switch statement is considered a looping structure for the purposes of continue.

continue accepts an optional numeric argument which tells it how many levels of enclosing loops it should skip to the end of.

 

<?php
while (list($key, $value) = each($arr)) {
if (!($key % 2)) { // skip odd members
continue;
}
do_something_odd($value);
}

$i = 0;
while ($i++ < 5) {
echo "Outer<br />\n";
while (1) {
echo "&nbsp;&nbsp;Middle<br />\n";
while (1) {
echo "&nbsp;&nbsp;Inner<br />\n";
continue 3;
}
echo "This never gets output.<br />\n";
}
echo "Neither does this.<br />\n";
}
?>

 

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