PHP call_user_func vs. just calling function

I'm sure there's a very easy explanation for this. What is the difference between this:

function barber($type){
    echo "You wanted a $type haircut, no problem\n";
}
call_user_func('barber', "mushroom");
call_user_func('barber', "shave");

... and this (and what are the benefits?):

function barber($type){
    echo "You wanted a $type haircut, no problem\n";
}
barber('mushroom');
barber('shave');

Always use the actual function name when you know it.

call_user_func is for calling functions whose name you don't know ahead of time but it is much less efficient since the program has to lookup the function at runtime.

Although you can call variable function names this way:

function printIt($str) { print($str); }

$funcname = 'printIt';
$funcname('Hello world!');

there are cases where you don't know how many arguments you're passing. Consider the following:

function someFunc() {
  $args = func_get_args();
  // do something
}

call_user_func_array('someFunc',array('one','two','three'));

It's also handy for calling static and object methods, respectively:

call_user_func(array('someClass','someFunc'),$arg);
call_user_func(array($myObj,'someFunc'),$arg);

the call_user_func option is there so you can do things like:

$dynamicFunctionName = "barber";

call_user_func($dynamicFunctionName, 'mushroom');

where the dynamicFunctionName string could be more exciting and generated at run-time. You shouldn't use call_user_func unless you have to, because it is slower.

With PHP 7 you can use the nicer variable-function syntax everywhere. It works with static/instance functions, and it can take an array of parameters. More info at https://trowski.com/2015/06/20/php-callable-paradox

$ret = $callable(...$params);

I imagine it is useful for calling a function that you don't know the name of in advance... Something like:

switch($value):
{
  case 7:
  $func = 'run';
  break;
  default:
  $func = 'stop';
  break;
}

call_user_func($func, 'stuff');

There is no benefits calling the function like that because I think it mainly used to call "user" function (like plugin) because editing core file is not good option. here are dirty example used by Wordpress

<?php
/* 
* my_plugin.php
*/

function myLocation($content){
  return str_replace('@', 'world', $content);
}

function myName($content){
  return $content."Tasikmalaya";
}

add_filter('the_content', 'myLocation');
add_filter('the_content', 'myName');

?>

...

<?php
/*
* core.php
* read only
*/

$content = "hello @ my name is ";
$listFunc = array();

// store user function to array (in my_plugin.php)
function add_filter($fName, $funct)
{
  $listFunc[$fName]= $funct;
}

// execute list user defined function
function apply_filter($funct, $content)
{
  global $listFunc;
  
  if(isset($listFunc))
  {
    foreach($listFunc as $key => $value)
    {
      if($key == $funct)
      {
        $content = call_user_func($listFunc[$key], $content);
      }
    }
  }
  return $content;
}

function the_content()
{
  $content = apply_filter('the_content', $content);
  echo $content;
}
 
?>

....

<?php
require_once("core.php");
require_once("my_plugin.php");

the_content(); // hello world my name is Tasikmalaya
?>

output

hello world my name is Tasikmalaya

in your first example you're using function name which is a string. it might come from outside or be determined on the fly. that is, you don't know what function will need to be run at the moment of the code creation.

When using namespaces, call_user_func() is the only way to run a function you don't know the name of beforehand, for example:

$function = '\Utilities\SearchTools::getCurrency';
call_user_func($function,'USA');

If all your functions were in the same namespace, then it wouldn't be such an issue, as you could use something like this:

$function = 'getCurrency';
$function('USA');

Edit: Following @Jannis saying that I'm wrong I did a little more testing, and wasn't having much luck:

<?php
namespace Foo {

	class Bar {
		public static function getBar() {
			return 'Bar';
		}
	}
	echo "<h1>Bar: ".\Foo\Bar::getBar()."</h1>";
	// outputs 'Bar: Bar'
	$function = '\Foo\Bar::getBar';
	echo "<h1>Bar: ".$function()."</h1>";
	// outputs 'Fatal error: Call to undefined function \Foo\Bar::getBar()'
	$function = '\Foo\Bar\getBar';
	echo "<h1>Bar: ".$function()."</h1>";
	// outputs 'Fatal error: Call to undefined function \foo\Bar\getBar()'
}

You can see the output results here: https://3v4l.org/iBERh it seems the second method works for PHP 7 onwards, but not PHP 5.6.