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.

At 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 or contain multiple expressions separated by commas. In expr2, all expressions separated by a comma are evaluated but the result is taken from the last part. 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 the numbers 1 through 10:

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

The above example will output:

1 2 3 4 5 6 7 8 9 10

<?php
for ($i = 1; ; $i++) {
    if ($i > 10) {
        break;
    }
    echo $i . " ";
}

The above example will output:

1 2 3 4 5 6 7 8 9 10

<?php
$i = 1;
for (; ; ) {
    if ($i > 10) {
        break;
    }
    echo $i . " ";
    $i++;
}

The above example will output:

1 2 3 4 5 6 7 8 9 10

<?php
for ($i = 1, $j = 0; $i <= 10; $j += $i, print $i . " ", $i++);

The above example will output:

1 2 3 4 5 6 7 8 9 10

Of course, the first example appears to be the nicest one (or perhaps the fourth), but you may find that being able to use empty expressions in for loops comes in handy on many occasions.

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

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

Expressions expr2 and expr3 are evaluated every iteration. It's advisable to use simple expressions in these places to avoid performance issues. For example, if the number of iterations is known in advance, it is better to use a variable instead of a function call in expr2:

<?php

$people = ['Kalle', 'Pierre'];

// Bad practice: calling a function on every iteration
for($i = 0; $i < getIterationCount($people); ++$i) {
    $people[$i] .= ' is cool';
}

// Good practice: storing the count in a variable
for($i = 0, $peopleCount = getIterationCount($people); $i < $peopleCount; ++$i) {
    $people[$i] .= ' is cool';
}

In the example above, the function is called on every iteration of the first loop, even though it always returns the same value. Storing that value in a variable, as the second loop does, calls the function once. Note that the number of iterations is then fixed: if the array is modified inside the loop, the stored value no longer reflects its size.