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:
/* example 1 */
for ($i = 1; $i <= 10; $i++) {
print $i;
}
/* example 2 */
for ($i = 1;;$i++) {
if ($i > 10) {
break;
}
print $i;
}
/* example 3 */
$i = 1;
for (;;) {
if ($i > 10) {
break;
}
print $i;
$i++;
}
/* example 4 */
for ($i = 1; $i <= 10; print $i, $i++);
|
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 in many
occasions.
PHP also supports the alternate "colon syntax" for for loops.
for (expr1; expr2; expr3): statement; ...; endfor;
|
Other languages have a foreach statement to traverse an array or hash. PHP 3 has
no such construct; PHP 4 does (see foreach). In PHP
3, you can combine while with the list() and each() functions
to achieve the same effect. See the documentation for these functions for an example.
