This function is great for input validation.  I frequently need to check that all characters in a string are 7-bit ASCII (and not null).  This is the fastest function I have found yet:

<?php
function is7bit($string) {
    // empty strings are 7-bit clean
    if (!strlen($string)) {
        return true;
    }
    // count_chars returns the characters in ascending octet order
    $str = count_chars($str, 3);
    // Check for null character
    if (!ord($str[0])) {
        return false;
    }
    // Check for 8-bit character
    if (ord($str[strlen($str)-1]) & 128) {
        return false;
    }
    return true;
}
?>