In ZE / PHP 4 objects were passed by reference, so when variables containing objects were passed to functions a local copy of the object was made for the function.

<?php
class foo {
     var $foo = 1, $bar = 10;

    function baz($val) {
        $this->bar *= $val;
    }
}

function mod_foo($foo)
{
    $foo->baz(10);
    $foo->foo = 123;
}

$a = new foo();
mod_foo($a);

echo $a->bar . "<br />"; // PHP4: 10 PHP5: 100
echo $a->foo . "<br />"; // PHP4: 1 PHP5: 123
?>