It prevents typos which cause very hard to debug bugs. That's in case you accidentally write =
instead of ==
:
if ($var == null) # This is true in case $var actually is null
if ($var = null) # This will always be true, it will assign $var the value null
Instead, switching them is safer:
if (null == $var) # True if $var is null
if (null = $var) # This will raise compiler (or parser) error, and will stop execution.
Stopping execution on a specific line with this problem will make it very easy to debug. The other way around is quite harder, since you may find yourself entering if
conditions, and losing variable's values, and you won't find it easily.