evil_mutable.php

<?php

/**
 * Return the object's value + 2
 */
function f( $obj ) {
	$retval = $obj->value + 2;
	$obj->value = 5; // EVIL!
	return $retval;
}

/**
 * Return the object's value + 3
 */
function g( $obj ) {
	return $obj->value + 3;
}

$myObj = new stdClass();
$myObj->value = 1;
echo f( $myObj ) + g( $myObj ); // Outputs 11, not 7