Closure vs Callable Type Hinting

• function foo(callable $bar) PHP 5.4+
• function foo(callable $bar = null) PHP 5.4+
• function foo(): callable PHP 7.0+
• function foo(): ?callable PHP 7.1+

When type hinting, a closure must be an anonymous function, where a callable also can be a normal method/function.

• Closure is a class and callable is a type.
• The callable type accepts anything that can be called

function callableFunc(Callable $callback) {
    $callback();
}

function closureFunc(Closure $closure) {
    $closure();
}

function anyFunc(Callable $callable) {
    $callable();
}

function testFunc() {
    echo 'Hello, World!', "
";
}

$testClosure = function(){
	echo 'Hello, World!', "
"; 
};
// gettype($testClosure);   object
// var_dump($testClosure);  object(Closure)#1 (0) { } 

try{
	closureFunc($testClosure); // Hello, World!
	callableFunc("testFunc"); // Hello, World!
	anyFunc($testClosure); // Hello, World!
	callableFunc($testClosure); // Hello, World!
	closureFunc("testFunc");// Argument 1 passed to closureFunc() must be an instance of Closure, string given
} 
catch(TypeError $e){
    echo $e->getMessage(), "
";
}
finally {
    echo "Reachable statement";
}