Here's the reproduce code:

<?php
error_reporting(E_ALL);

if (version_compare(PHPVERSION(), '4.4.0') != 0) print "This reproduce code is only for PHP 4.4.0";

$bar = new Bug();
$baz = $bar->foo();

class Bug {
    var $_flag;
    function Bug() {
	$this->_flag = 0;
    }

    function &foo() {
	if (!$this->_flag) {
	    $false = false;
	    return $false; /* this is line 18 */
	}

	$false = false;
	return $false;
    }
}
?>

Expected result:
----------------
no NOTICEs

Actual result:
--------------
Notice: Only variable references should be returned by reference in
reproduce.php on line 18

I've added the two other methods in the original reproduce code just to show that very slight modifications of the code do not give a PHP Notice.
And please remember that I can't reproduce it on all systems.

John Lim from the adodb project wrote the following:
> There is nothing wrong with this code.
> 
> I have found this problem normally occurs when php 4.4 is
> being run with an old version of zend optimizer or similar 
> 3rd party software.

Also, one of the ISPs that can reproduce this incorrect behavior said they've compiled PHP statically, but zend was   DSO. They use the same PHP on multiple boxes and only on one they can reproduce this issue. I'll notify them about the statement from John Lim, probably that's it.

If that's the case, the bug is indeed invalid. But it's good to know what can cause this issue.

The bug here is in Zend Optimizer.  It sees your variable assigned immediately before the return and substitutes the intermediate TMP_VAR for the real variable.

Please report this bug to Zend at http://www.zend.com