Why can I pass a callback function without quotation marks in PHP5? - php

Question

I use the callback function without quotation marks, although it returns an error, it can be executed normally.
function fc($v) {
echo $v + 1;
}
$a = [2, 4];
array_map(fc, $a); // <-- this works! notice how fc is not a string.
// output: 35
Although it seems useless, I want to know why it works.

Answer 1

PHP converts undefined constants to strings automagically.
If you let PHP execute this:
echo my_undefined_constant . " is a " . gettype(my_undefined_constant);
PHP will complain, a lot, but eventually my_undefined_constant will be the string "my_undefined_constant":
Warning: Use of undefined constant my_undefined_constant - assumed 'my_undefined_constant' (this will throw an Error in a future version of PHP) in php shell code on line 1
Call Stack:
136.9851 395312 1. {main}() php shell code:0
Warning: Use of undefined constant undefined_constant - assumed 'undefined_constant' (this will throw an Error in a future version of PHP) in php shell code on line 1
Call Stack:
136.9851 395312 1. {main}() php shell code:0
my_undefined_constant is a string <---
But, as a result of undefined constants being converted to string literals, this will work:
php > echo call_user_func(strlen, 'test123');
Warning: Use of undefined constant strlen - assumed 'strlen' (this will throw an Error in a future version of PHP) in php shell code on line 1
Call Stack:
258.5445 395240 1. {main}() php shell code:0
7 <---
This behavior is obviously highly questionable, and that's why implicitly converting undefined constants to string literals has been deprecated since PHP 7.2.

Answer 2

Its php feature that for warning, notice errors it displays them but continue executing script.
In similar manner though it has thrown error(warning) array_map function did its job.