Why do different delivery methods have different results when applying PHP's global keyword?

Question

PHP implements the static and global modifier for variables in terms of references. For example, a true global variable imported inside a function scope with the global statement actually creates a reference to the global variable. This can lead to unexpected behaviour which the following example addresses:

<?php
function test_global_ref() {
    global $obj;
    $new = new stdClass;
    $obj =& $new;
}

function test_global_noref() {
    global $obj;
    $new = new stdClass;
    $obj = $new;
}

test_global_ref();
var_dump($obj);
test_global_noref();
var_dump($obj);
?>

The above example will output:

NULL
object(stdClass)#1 (0) {
}

URL: https://www.php.net/manual/en/language.variables.scope.php#language.variables.scope.references

I found this example on the variable scope page of the PHP manual, but I don't understand why the first var_dump is null. I even output $obj in the test_global_ref function. $obj still has value, but why is it null when outputting $obj in the global scope?

Answer 1

In the first code sample, $obj is set as a reference to $new. When that function ends, $new is cleaned up (as it's no longer in scope for any actively running code), leaving the reference going nowhere.

In the second sample, $obj is set to $new without using a reference, so PHP knows to keep the value around.