This comment at php.net contains a valuable piece of wisdom:

If you need to compare a variable with a value, instead of doing:
<?php
  if ($foo == 3) bar();
?>

do:
<?php
  if (3 == $foo) bar();
?>

This way, if/when you forget the second equals sign (=), the statement will become:
<?php
  if (3 = $foo) bar();
?>

and PHP will evaluate the statement and report an error.