The for loop has three semicolon-separated expressions within its parentheses. These three expressions function respectively as the initialization, the condition, and the reinitialization expressions of the loop. The for loop can be defined in terms of the corresponding while loop:

for ($i = 1; $i < 10; $i++) {
    ...
}

This is the same as:

$i = 1;
while ($i < 10) {
    ...
}
continue {
    $i++;
}