In PHP, functions "are" "first" "class" "objects"

first_class_fns.php

// Psy Shell v0.4.4 (PHP 5.6.4-4ubuntu6.2 — cli) by Justin Hileman
>>> function foo($x) { return $x * 2; }
=> null
>>> foo(5)
=> 10
>>> is_callable(foo)
PHP error:  Use of undefined constant foo - assumed 'foo' on line 1
>>> is_callable('foo')
=> true
>>> 'foo'(5)
PHP Parse error: Syntax error, unexpected '(' on line 1
>>> ('foo')(5)
PHP Parse error: Syntax error, unexpected '(' on line 1
>>> $bar = 'foo';
=> "foo"
>>> is_callable($bar)
=> true
>>> $bar(5)
=> 10
>>> is_callable('array_map')
=> true
>>> $map = 'array_map';
=> "array_map"
>>> $map($bar, [1, 2, 3])
=> [
       2,
       4,
       6
   ]
>>>