PHP Variable names

Variable names always begin with a dollar sign ($) and are case-sensitive. Here are
some valid variable names:
$pill
$ad_count
$dForce
$I_kk_PHP
$_underscore
$_int
Here are some illegal variable names:
$not valid
$|
$3ka 
These variables are all different:
$hot_stuff $Hot_stuff $hot_Stuff $HOT_STUFF