双重等于for循环？

Question

Why doesn't the double equals work in a PHP for loop? The following works:

$cnt = 5;
for ($z = 0; $z <= $cnt; $z++) {
    echo $z."!<br />";
}

But why doesn't this work?

$cnt = 5;
for ($z = 0; $z == $cnt; $z++) {
    echo $z."!<br />";
}

Answer 1

The loop executes only if the condition evaluates to true. At the first iteration, $z == $cnt is false, so the loop never executes.

A common loop strategy is to use a sentinel value:

$cnt = 5;
$stop = $cnt + 1;
for ($z = 0; $z != $stop; $z++) {
    . . .
}

Note that the comparison is negative (!= or !==). Using a sentinel is usually unnecessary for numerical loop variables (I wouldn't recommend it for your posted code), but is useful for other situations (e.g., when $stop represents null, an illegal value, etc.). It's particularly helpful when the loop variable changes in a pattern that is not easy to characterize succinctly.