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Variables are fundamental to all programming languages. They are data items that represent a memory storage location in the computer. Variables are containers that hold data such as numbers and strings. In PHP programs there are three types of variables:
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Variables have a name, a type, and a value.
The values assigned to variables can change throughout the run of a program whereas constants, also called literals, remain fixed.
PHP variables can be assigned different types of data, including:
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Computer programming languages like C++ and Java require that you specify the type of data you are going to store in a variable when you declare it. For example, if you are going to assign an integer to a variable, you would have to say something like:
and if you were assigning a floating-point number:
Languages that require that you specify a data type are called "strongly typed" languages. PHP, conversely, is a dynamically, or loosely typed, language, meaning that you do not have to specify the data type of a variable. In fact, doing so will produce an error. With PHP you would simply say:
and PHP will figure out what type of data is being stored in $n and $x.
Variable names consist of any number of letters (an underscore counts as a letter) and digits. The first letter must be a letter or an underscore (see Table 4.2). Variable names are case sensitive, so Name, name, and NAme are all different variable names.
Table 4.2 Valid and Invalid Variable Name Examples
| Valid Variable Names | Invalid Variable Names |
| $name1 | $10names |
| $price_tag | box.front |
| $_abc | $name#last |
| $Abc_22 | A-23 |
| $A23 | $5 |
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