what does a php function return by default?

You can try it out by doing:

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

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!)

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