PHP Variables

Variables in PHP

Variables are used for storing values, like text strings, numbers or arrays.  When a variable is declared, it can be used over and over again in your script. All variables in PHP start with a $ sign symbol. The correct way of declaring a variable in PHP:

$var_name = value;

New PHP programmers often forget the $ sign at the beginning of the variable. In that case it will not work.
Let's try creating a variable containing a string, and a variable containing a number:

<?php
$txt="Hello World!";
$x=16;
?> 

PHP is a Loosely Typed Language

In PHP, a variable does not need to be declared before adding a value to it. In the example above, you see that you do not have to tell PHP which data type the variable is. PHP automatically converts the variable to the correct data type, depending on its value. In a strongly typed programming language, you have to declare (define) the type and name of the variable before using it. In PHP, the variable is declared automatically when you use it.

Naming Rules for Variables

• A variable name must start with a letter or an underscore "_"
• A variable name can only contain alpha-numeric characters and underscores (a-z, A-Z, 0-9, and _ )
• A variable name should not contain spaces. If a variable name is more than one word, it should be separated with an underscore ($my_string), or with capitalization ($myString)

http://www.w3schools.com/php/php_variables.asp