The main way to store information in the middle of a PHP program is by using a variable.
The most important things to know about variables in PHP
All variables in PHP are denoted with a leading dollar sign ($).
The value of a variable is the value of its most recent assignment.
Variables are assigned with the = operator, with the variable on the left-hand side and the expression to be evaluated on the right.
Variables can, but do not need, to be declared before assignment.
Variables in PHP do not have intrinsic types - a variable does not know in advance whether it will be used to store a number or a string of characters.
Variables used before they are assigned have default values.
PHP does a good job of automatically converting types from one to another when necessary.
PHP variables are Perl-like.
See this example:
Following is the example to store string, integer and float values in PHP variables.
In PHP, variable names are case sensitive. So variable name "book" is different from Book, BOOK, BoOk etc.
PHP variables must start with letter or underscore only.
PHP variable can't be start with numbers and special symbols.
Invalid PHP Variables