I've been developing in PHP for a while now, mostly iZeit, my PHP calendar script, but also the odd smaller script and I've picked up a few things along the way.

Quotes vs. Double Quotes

Anything in double quotes is parsed by PHP and variables are replaced. So if you’re concatenating things, use single quotes instead:

$name = 'Bob';
echo 'Name is '.$name;   // Name is Bob
echo "Name is $name";   // Name is Bob
echo 'Name is $name';   // Name is $name
echo "Name is ".$name;   /* Name is Bob, but wastes processing
time because the non dynamic part of the statement is parsed for variables. */

Check whether a variable's set first

There are times when if you try to use a variable that isn't set, PHP will throw an error at you.

$param = isset($_GET['param']) ? $_GET['param'] : NULL;

Using shorttags

I have no idea why they even added shorttags. Using <?php ?> instead of <? ?> will make your scripts run on alot more servers which don't have short tags enabled in php.ini.