<?php
function my_callback( &$param ) {}
$callback = 'my_callback';
$a = true;
$b = array( $a );
$c = array( &$a );
//my_callback( true ); // Fatal error: passing a value when reference is expected.
my_callback( $a ); // OK
//$callback( true ); // Fatal error: passing a value when reference is expected.
$callback( $a ); // OK
call_user_func( $callback, true ); // Warning: Parameter 1 to my_callback() expected to be a reference, value given
call_user_func( $callback, $a ); // Warning: Parameter 1 to my_callback() expected to be a reference, value given
call_user_func( $callback, ...$b ); // Warning: Parameter 1 to my_callback() expected to be a reference, value given
call_user_func( $callback, ...$c ); // Warning: Parameter 1 to my_callback() expected to be a reference, value given
call_user_func_array( $callback, array( true ) ); // Warning: Parameter 1 to my_callback() expected to be a reference, value given
call_user_func_array( $callback, array( $a ) ); // Warning: Parameter 1 to my_callback() expected to be a reference, value given
call_user_func_array( $callback, $b ); // Warning: Parameter 1 to my_callback() expected to be a reference, value given
call_user_func_array( $callback, array( &$a ) ); // OK
call_user_func_array( $callback, $c ); // OK