what does a php function return by default?

If I return nothing explicitly, what does a php function exactly return?

function foo() {}

1. What type is it?

2. What value is it?

3. How do I test for it exactly with === ?

4. Did this change from php4 to php5?

5. Is there a difference between function foo() {} and function foo() { return; }

(I am not asking how to test it like if (foo() !=0) ...)

Solution 1:

1. null
2. null
3. if(foo() === null)
4. -
5. Nope.

You can try it out by doing:

$x = foo();
var_dump($x);

Solution 2:

Not returning a value from a PHP function has the same semantics as a function which returns null.

function foo() {}

$x=foo();

echo gettype($x)."\n";
echo isset($x)?"true\n":"false\n";
echo is_null($x)?"true\n":"false\n";

This will output

NULL
false
true

You get the same result if foo is replaced with

function foo() {return null;}

There has been no change in this behaviour from php4 to php5 to php7 (I just tested to be sure!)