In addition to the note made by "Francis dot a at gmx dot net" you should not normally be using a function such as sizeof() or count() in a control structure such as FOR because the same value is being calculated repeatedly for each iteration. This can slow things down immensely, regardless of whether you pass by value or reference.
It is generally much better to calculate the static values before the defining the looping control structure.
Example:
<?php
$intSize = sizeof($arrData);
for($i = 0; $i < $intSize; $n++) {
}
?>
