$b is assigned a copy of $a, and in the second part, $b is assigned a reference to $a.
<?php $a = 5; $b = $a; $a = 7; echo "\$a = $a and \$b = $b\n"; $a = 5; $b = &$a; $a = 7; echo "\$a = $a and \$b = $b\n"; ?>