Don't mix comparison (==) with assignment (=)

When using comparisons always put your constants on the left. This practice will help you avoid mixing comparison (==) with assignment (=) and will save you hours of clueless debugging.

Recommended:

if (null == $var) {}

if (15 == $number) {}

Not recommended:

if ($var == null {}

if ($number == 15) {}

Bad (!!):
if ($var = null {}

if ($number = 15) {}

This is something you might find a bit hard to get use to, but at the end should probably pay off.