PHP Variable

PHP also have a variable to store temporary data. Just like Javascript variable, PHP variable can store anykind of data in 1 variable. For example, you can store 234 (number) in a variable and then change it to "Hello" (string) later.
PHP have some rule if you want to declare a variable:

1. a name must start with letter or underscore "_"
2. a variable name must contain only alpha-numeric characters or underscore (a-z, A-Z, 0-9, _ )
3. we cannot use space for variable name but we can use _ or with capitalization ($my_var or $myVar) 
4. php variable is case sensitive