pass_param_array.php

<?php

function change_value_1(array $o) {
  $o['b'] = 5;
}

function change_value_2(array $o) {
  $o = array();
  $o['b'] = 5;
}

function change_value_3(array &$o) {
  $o = array();
  $o['b'] = 5;
}

function test($func_name) {
  $o = array('a' => 1, 'b' => 2);
  call_user_func($func_name, $o);

  print_r($o);
}

test('change_value_1');
// => array('a' => 1, 'b' => 2);

test('change_value_2');
// => array('a' => 1, 'b' => 2);

//test('change_value_3');
// this will result a warning "Parameter 1 to change_value_3() expected to be a reference, value given"

$o = array('a' => 1, 'b' => 2);
change_value_3($o);
print_r($o);
// => array('b' => 5);

pass_param_obj.php

<?php

class Obj {
  public $value;

  public function __construct() {
    $this->value = 1;
  }
}

function change_value_1(Obj $o) {
  $o->value = 5;
}

function change_value_2(Obj $o) {
  $o = new Obj();
  $o->value = 5;
}

function change_value_3(Obj &$o) {
  $o = new Obj();
  $o->value = 5;
}

function test($func_name) {
  $o = new Obj();
  call_user_func($func_name, $o);

  echo $o->value . "\n";
}

test('change_value_1');
// => 5

test('change_value_2');
// => 1

//test('change_value_3'); 
// this will result a warning "Parameter 1 to change_value_3() expected to be a reference, value given"


$o = new Obj();
change_value_3($o);
echo $o->value . "\n";
// => 5