Very little about PHP's treatment of functions is easy or clean.

    function foo() { echo "Hello world"; }

    $w = 'foo';
    echo $w;  // foo
    $w();     // Hello world
    'foo'();  // PHP Parse error: syntax error, unexpected '('

    function bar() { return 'foo'; }
    $x = bar();
    echo $x;  // foo
    $x();     // Hello world
    bar()();  // PHP Parse error:  syntax error, unexpected '('

    $o = new stdClass;
    $o->baz = 'foo';
    $y = $o->baz;
    echo $y;  // foo
    $y();     // Hello world
    $o->baz();  // PHP Fatal error:  Call to undefined method stdClass::baz()

    $z = function() {
           echo "Hello world";
         };
    $z();  // Hello world
    (function() {
       echo "Hello world";
     })();  // PHP Parse error:  syntax error, unexpected '('