PHP 变量

In this tutorial you will learn how store information in a variable in PHP.

## What is Variable in PHP
Variables are used to store data, like string of text, numbers, etc. Variable values can change over the course of a script. Here're some important things to know about variables:

- In PHP, a variable does not need to be declared before adding a value to it. PHP automatically converts the variable to the correct data type, depending on its value.
- After declaring a variable it can be reused throughout the code.
- The assignment operator (`=`) used to assign value to a variable.

In PHP variable can be declared as: `$var_name = value;`

```php
<?php
// Declaring variables
$txt = "Hello World!";
$number = 10;
 
// Displaying variables value
echo $txt;  // Output: Hello World!
echo $number; // Output: 10
?>
```
In the above example we have created two variables where first one has assigned with a string value and the second has assigned with a number. Later we've displayed the variables values in the browser using the `echo` statement. The PHP [echo statement](http://www.bixiaguangnian.com/manual/php7/3968.html "echo statement") is often used to output data to the browser. We will learn more about this in upcoming chapter.

## Naming Conventions for PHP Variables
These are the following rules for naming a PHP variable:

- All variables in PHP start with a `$` sign, followed by the name of the variable.
- A variable name must start with a letter or the underscore character `_`.
- A variable name cannot start with a number.
- A variable name in PHP can only contain alpha-numeric characters and    underscores (`A-z`, `0-9`, and `_`).
- A variable name cannot contain spaces.

<div class="callout callout-info mb-3">Note: Variable names in PHP are case sensitive, it means $x and $X are two different variables. So be careful while defining variable names.</div>