Variables are used to store data temporarily in memory. Variables only exist as long as the PHP script is running. They are identified with a dollar sign followed by a name. The name of a variable can begin with a letter (a-z or A-Z) or an underscore. Any of the following characters can be a number, a letter, or any ASCII character from 127 to 255 (0x7f – 0xff). There is no restriction on the number of characters in the name. Variable names are case sensitive so $var, $Var, and $VAR will be three different variables.