PHP Variables are used for storing data such as numeric values, characters, character strings etc. All PHP variable names must be pre-fixed with a ( $). Variable names are case-sensitive they are ($var and $VAR are two different variables).
Syntax of declaring a PHP valid and invalid variable names :
$varName // valid
$_varName // valid
$__varname // valid
$var_name // valid
$varName21 // valid
$_var-Name // invalid contains non alphanumeric character (-)
$_9Var // invalid - underscore must be followed by a letter
$99Var // invalid - must begin with a letter or underscore
