$functionName() or call_user_func($functionName)

If you need to provide parameters stored in another variable (in the form of array), use array unpacking operator:

$function_name = 'trim';
$parameters = ['aaabbb','b'];
echo $function_name(...$parameters); // aaa

To dynamically create an object and call its method use

$class = 'DateTime';
$method = 'format';
echo (new $class)->$method('d-m-Y');

or to call a static method

$class = 'DateTime';
$static = 'createFromFormat';
$date = $class::$static('d-m-Y', '17-08-2023');

Yes, you can do ajax request to server with your data in request parameters, like this (very simple):

Note that the following code uses jQuery

jQuery.ajax({
    type: "POST",
    url: 'your_functions_address.php',
    dataType: 'json',
    data: {functionname: 'add', arguments: [1, 2]},

    success: function (obj, textstatus) {
                  if( !('error' in obj) ) {
                      yourVariable = obj.result;
                  }
                  else {
                      console.log(obj.error);
                  }
            }
});

and your_functions_address.php like this:

    <?php
    header('Content-Type: application/json');

    $aResult = array();

    if( !isset($_POST['functionname']) ) { $aResult['error'] = 'No function name!'; }

    if( !isset($_POST['arguments']) ) { $aResult['error'] = 'No function arguments!'; }

    if( !isset($aResult['error']) ) {

        switch($_POST['functionname']) {
            case 'add':
               if( !is_array($_POST['arguments']) || (count($_POST['arguments']) < 2) ) {
                   $aResult['error'] = 'Error in arguments!';
               }
               else {
                   $aResult['result'] = add(floatval($_POST['arguments'][0]), floatval($_POST['arguments'][1]));
               }
               break;

            default:
               $aResult['error'] = 'Not found function '.$_POST['functionname'].'!';
               break;
        }

    }

    echo json_encode($aResult);

?>

If you are using a class, then you can use $this for calling the function:

class Test {

    public function say($a) {
        return $a ;

    } 

    public function tell() {
        $c = "Hello World" ;
        $a = $this->say($c) ;
        return $a ;
    }
} 

$b= new Test() ;    
echo $b->tell() ;

If you are using a normal function, then use closure:

function tell(){
   $a = "Hello" ;
   return function($b) use ($a){
      return $a." ".$b ;
   } ;  
}

$s = tell() ;
echo $s("World") ; 

The simplest way is:

echo debug_backtrace()[1]['function'];

As noted in the comments below, this can be further optimized by passing arguments to:

• omit both the object and args indices
• limit the number of stack frames returned

echo debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS,2)[1]['function'];

Define the function above your return value, otherwise it never gets executed.

<?php
function foo($i){
    function bar($i){
        return $i*4;
    }
    return bar($i)*4;
}
echo foo(4);
?>