I just noticed the following behavior in my php code and I'm wondering why it's happening.
$array = array();
test_value($array['invalid_index']); // Error -> Notice: Undefined index: invalid_index in ...
test_reference($array['invalid_index']); //No error
Function parameters by-reference accept variables which haven't been declared previously; they are being declared by passing them as reference. This makes complete sense if you look at typical use cases of by-reference parameters:
$foo = preg_match('/(.)/', 'bar', $baz);
This function returns one value, which will be assigned to
$foo. However, this function should also return the matches, which here is the third parameter
$baz. Since a function can only
return a single value at a time, additional "return" values are being realised by populating variables passed by reference.
If you had to pre-initialise these variables, your code would look like this:
$baz = null; $foo = preg_match('/(.)/', 'bar', $baz); echo $baz;
This makes code more complicated than necessary; PHP hence doesn't require that such variables "exist" beforehand.