<?php
function utime() { 
    list($usec, $sec) = explode(" ",microtime()); 
    return ((float)$usec + (float)$sec);
}

function large_array_diff($b, $a) {
    $at = array();
    foreach ($a as $i)
        $at[$i] = 1;

    $d = array();

    foreach ($b as $i)
        if (!isset($at[$i]))
            $d[] = $i;

    return $d;
}

function flip_isset_diff($b, $a) {
    $at = array_flip($a);
    $d = array();
    foreach ($b as $i)
        if (!isset($at[$i])) 
            $d[] = $i;
    return $d;
}

function flip_array_diff_key($b, $a) {
    $at = array_flip($a);
    $bt = array_flip($b);
    $d = array_diff_key($bt, $at);
    return array_keys($d);
}

$a = file("l.a"); // custom file contains 2.5 millions of lines
$b = file("l.b"); // custom file contains 2.5 millions of lines


$methods = array(
    //'array_diff' => 'PHPs internal array_diff',
    'large_array_diff' => 'Complete Custom',
    'flip_isset_diff' => 'Flip one and isset',
    'flip_array_diff_key' => 'Flip both and array_diff_key'
);
$times = array();
foreach ($methods as $k => $v) {
    $start = utime();
    call_user_func($k, $a, $b);
    $tm = utime()- $start;
    $times[$k] = $tm;
}

fprintf(STDOUT, "%s--%s\n", str_repeat('-', 30), str_repeat('-', 20));
fprintf(STDOUT, "%-30s  %s\n", "Name", "Execution Time");
fprintf(STDOUT, "%s--%s\n", str_repeat('-', 30), str_repeat('-', 20));
foreach($methods as $k => $v){
    fprintf(STDOUT, "%s--%s\n", str_repeat('-', 30), str_repeat('-', 20));
    fprintf(STDOUT, "%-30s  %03.3f\n", $v, $times[$k]);
}
fprintf(STDOUT, "%s--%s\n", str_repeat('-', 30), str_repeat('-', 20));
?>