Test the speed of calling a function in various ways in PHP.

func_speed.php

<?php

function fooBar($hello, $world) {
    // Nothing to see here...
}

$results = array();


// TEST 1: call_user_func
$times = array();

for ($i=0; $i<10000; $i++) {
    $start = microtime(true);
    
    call_user_func('fooBar', 'hello', 'world');
    
    $end = microtime(true);
    
    $times[] = ($end - $start);
}

asort($times);

$results['call_user_func'] = array(
    array_sum($times) / count($times),
    current(array_slice($times, 0, 1)),
    current(array_slice($times, -1, 1)),
    array_sum($times)
);


// TEST 2: call_user_func_array
$times = array();

for ($i=0; $i<10000; $i++) {
    $start = microtime(true);
    
    call_user_func_array('fooBar', array('hello', 'world'));
    
    $end = microtime(true);
    
    $times[] = ($end - $start);
}

asort($times);

$results['call_user_func_array'] = array(
    array_sum($times) / count($times),
    current(array_slice($times, 0, 1)),
    current(array_slice($times, -1, 1)),
    array_sum($times)
);


// TEST 3: native
$times = array();

for ($i=0; $i<10000; $i++) {
    $start = microtime(true);
    
    fooBar('hello', 'world');
    
    $end = microtime(true);
    
    $times[] = ($end - $start);
}

asort($times);

$results['native'] = array(
    array_sum($times) / count($times),
    current(array_slice($times, 0, 1)),
    current(array_slice($times, -1, 1)),
    array_sum($times)
);


// TEST 4: variable function name
$times = array();

for ($i=0; $i<10000; $i++) {
    $start = microtime(true);
    
    $fooBar = 'fooBar';
    $fooBar('hello', 'world');
    
    $end = microtime(true);
    
    $times[] = ($end - $start);
}

asort($times);

$results['variable'] = array(
    array_sum($times) / count($times),
    current(array_slice($times, 0, 1)),
    current(array_slice($times, -1, 1)),
    array_sum($times)
);


// TEST 5: ReflectionFunction->invoke()
$times = array();

for ($i=0; $i<10000; $i++) {
    $start = microtime(true);
    
    $reflection = new ReflectionFunction('fooBar');
    $reflection->invoke('hello', 'world');
    
    $end = microtime(true);
    
    $times[] = ($end - $start);
}

asort($times);

$results['ReflectionFunction->invoke'] = array(
    array_sum($times) / count($times),
    current(array_slice($times, 0, 1)),
    current(array_slice($times, -1, 1)),
    array_sum($times)
);


// TEST 6: ReflectionFunction->invoke()
$times = array();

for ($i=0; $i<10000; $i++) {
    $start = microtime(true);
    
    $reflection = new ReflectionFunction('fooBar');
    $reflection->invokeArgs(array('hello', 'world'));
    
    $end = microtime(true);
    
    $times[] = ($end - $start);
}

asort($times);

$results['ReflectionFunction->invokeArgs'] = array(
    array_sum($times) / count($times),
    current(array_slice($times, 0, 1)),
    current(array_slice($times, -1, 1)),
    array_sum($times)
);


// TEST 7: variable with a swtich statement
$times = array();

for ($i=0; $i<10000; $i++) {
    $start = microtime(true);

    $func = 'fooBar';
    $args = array('hello', 'world');

    switch (count($args)) {
        case 0:
            $func();
            break;
        case 1:
            $func($args[0]);
            break;
        case 2:
            $func($args[0], $args[1]);
            break;
        case 3:
            $func($args[0], $args[1], $args[2]);
            break;
        case 4:
            $func($args[0], $args[1], $args[2], $args[3]);
            break;
    }
    
    $end = microtime(true);
    
    $times[] = ($end - $start);
}

asort($times);

$results['variable switch'] = array(
    array_sum($times) / count($times),
    current(array_slice($times, 0, 1)),
    current(array_slice($times, -1, 1)),
    array_sum($times)
);


// TEST 8: variable with list
$times = array();

for ($i=0; $i<10000; $i++) {
    $start = microtime(true);

    $func = 'fooBar';
    $args = array('hello', 'world');

    $argc = count($args);
    $args = $args + array_fill($argc, 10 - $argc, null);
    
    list($a, $b, $c, $d, $e, $f, $g, $h, $j, $k) = $args;
    
    $func($a, $b, $c, $d, $e, $f, $g, $h, $j, $k);
    
    $end = microtime(true);
    
    $times[] = ($end - $start);
}

asort($times);

$results['variable list'] = array(
    array_sum($times) / count($times),
    current(array_slice($times, 0, 1)),
    current(array_slice($times, -1, 1)),
    array_sum($times)
);



asort($results);

foreach ($results as $key => $value) {
    array_walk($value, function(&$v, $k) {
        $v = number_format($v, 10);
    });
    $spaces = str_repeat(' ', 40 - strlen($key));
    print $key . ":" . $spaces . implode('     ', $value) . "\n";
}

gistfile1.txt

native:                                  0.0000018658     0.0000009537     0.0000119209     0.0186583996
variable:                                0.0000020720     0.0000009537     0.0000190735     0.0207200050
call_user_func:                          0.0000032218     0.0000019073     0.0000691414     0.0322182178
call_user_func_array:                    0.0000034505     0.0000028610     0.0002079010     0.0345051289
variable switch:                         0.0000036948     0.0000028610     0.0000197887     0.0369477272
ReflectionFunction->invoke:              0.0000049277     0.0000038147     0.0000329018     0.0492768288
ReflectionFunction->invokeArgs:          0.0000052142     0.0000047684     0.0000150204     0.0521419048
variable list:                           0.0000058471     0.0000047684     0.0000200272     0.0584714413


PHP 5.5.8 (cli) (built: Jan 12 2014 18:50:29)
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2013 Zend Technologies
    with Zend OPcache v7.0.3-dev, Copyright (c) 1999-2013, by Zend Technologies
    with Xdebug v2.2.3, Copyright (c) 2002-2013, by Derick Rethans

I have slightly improved your test.
We get interesting results when a variable number of arguments :)

On php 7:

native________: 0 - 0.0000000680 1 - 0.0000000597 2 - 0.0000000583 3 - 0.0000000789 4 - 0.0000000792
call_user_func: 0 - 0.0000000652 1 - 0.0000000673 2 - 0.0000000670 3 - 0.0000000898 4 - 0.0000000946

Call_user_func works faster than the native call if the number of arguments is 0.

native:                                  0.0000000492     0.0000000000     0.0000011921     0.0004920959
call_user_func:                          0.0000000560     0.0000000000     0.0000081062     0.0005598068
call_user_func_array:                    0.0000000615     0.0000000000     0.0000011921     0.0006153584
variable:                                0.0000000975     0.0000000000     0.0000081062     0.0009751320
variable switch:                         0.0000001495     0.0000000000     0.0000040531     0.0014946461
ReflectionFunction->invoke:              0.0000002904     0.0000000000     0.0000150204     0.0029041767
ReflectionFunction->invokeArgs:          0.0000003242     0.0000000000     0.0000090599     0.0032415390
variable list:                           0.0000004726     0.0000000000     0.0000090599     0.0047261715

PHP 7.4.9 (cli) (built: Oct 26 2020 15:17:14) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies
with Zend OPcache v7.4.9, Copyright (c), by Zend Technologies

native:                                  0.0000000455     0.0000000000     0.0000011921     0.0004551411
call_user_func:                          0.0000000517     0.0000000000     0.0000011921     0.0005173683
call_user_func_array:                    0.0000000677     0.0000000000     0.0000050068     0.0006768703
variable:                                0.0000000914     0.0000000000     0.0000240803     0.0009143353
variable switch:                         0.0000001434     0.0000000000     0.0000050068     0.0014338493
ReflectionFunction->invoke:              0.0000002131     0.0000000000     0.0000030994     0.0021307468
ReflectionFunction->invokeArgs:          0.0000002367     0.0000000000     0.0000100136     0.0023670197
variable list:                           0.0000004477     0.0000000000     0.0000081062     0.0044770241

PHP 8.1.13 (cli) (built: Nov 26 2022 14:07:36) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.13, Copyright (c) Zend Technologies
with Zend OPcache v8.1.13, Copyright (c), by Zend Technologies

native:                                  0.0000000275     0.0000000000     0.0000011921     0.0002751350
call_user_func_array:                    0.0000000299     0.0000000000     0.0000011921     0.0002989769
call_user_func:                          0.0000000306     0.0000000000     0.0000011921     0.0003063679
variable:                                0.0000000404     0.0000000000     0.0000011921     0.0004036427
variable switch:                         0.0000000625     0.0000000000     0.0000298023     0.0006246567
ReflectionFunction->invoke:              0.0000000826     0.0000000000     0.0000021458     0.0008258820
ReflectionFunction->invokeArgs:          0.0000000932     0.0000000000     0.0000011921     0.0009319782
variable list:                           0.0000001759     0.0000000000     0.0000011921     0.0017585754

PHP 8.2.17 (cli) (built: Mar 16 2024 01:08:19) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.2.17, Copyright (c) Zend Technologies
with Zend OPcache v8.2.17, Copyright (c), by Zend Technologies

native:                                  0.0000000258     0.0000000000     0.0000011921     0.0002579689
call_user_func:                          0.0000000297     0.0000000000     0.0000011921     0.0002968311
call_user_func_array:                    0.0000000302     0.0000000000     0.0000011921     0.0003015995
variable:                                0.0000000415     0.0000000000     0.0000011921     0.0004150867
variable switch:                         0.0000000607     0.0000000000     0.0000011921     0.0006067753
ReflectionFunction->invoke:              0.0000001019     0.0000000000     0.0000119209     0.0010190010
ReflectionFunction->invokeArgs:          0.0000001090     0.0000000000     0.0000021458     0.0010895729
variable list:                           0.0000001727     0.0000000000     0.0000059605     0.0017273426

PHP 7.4.28 (cli) (built: Mar 29 2022 03:23:10) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies