Note about uninitialized variables (#19289)

* Update index.md

Note about uninitialized variables.

* Update index.md

client/src/pages/guide/english/php/PHP Variables/index.md

@@ -1,31 +1,30 @@
----
-title: PHP Variables
----
-
-### PHP - Vaiables Types
-
-Varibles are the main way to store information in the middle way of a PHP program.
-All variables in PHP are donated with a leading dollar sign like `$variable_name`.
-Variables are assigned with the `= operator` , with the variable on the left-hand side and the expression to be evaluated on the right.
-
-### Variable Naming
-
-Rules for naming a variable is listed below:
- 1. Variables names must begin with a letter or underscores character.
- 2. A variable name can consist of numbers, letters, underscores but you cannot use characters like `+ , - , % , ( , ) . &` in its name.
- 3. Variable names are case-sensitive i.e. `($age and $AGE are two different variables)`.
-
-### Creating (Declaring) PHP Variables
-
-In PHP, a variable starts with the $ sign, followed by the name of the variable. The code snippet given below shows it.
-
-``` shell
- <?php
-    $txt = "Hello world!";
-    $x = 6;
-    $y = 10.5;
- ?>
- ```
- 
-Variable lifecycle: Before a variable is used, it has no existence. It is unset. So you can't use it by writing isset($variable) before using it.
- 
+---
+title: PHP Variables
+---
+
+### PHP - Vaiables Types
+
+Varibles are the main way to store information in the middle way of a PHP program.
+All variables in PHP are donated with a leading dollar sign like `$variable_name`.
+Variables are assigned with the `= operator` , with the variable on the left-hand side and the expression to be evaluated on the right.
+
+### Variable Naming
+
+Rules for naming a variable is listed below:
+ 1. Variables names must begin with a letter or underscores character.
+ 2. A variable name can consist of numbers, letters, underscores but you cannot use characters like `+ , - , % , ( , ) . &` in its name.
+ 3. Variable names are case-sensitive i.e. `($age and $AGE are two different variables)`.
+
+### Creating (Declaring) PHP Variables
+
+In PHP, a variable starts with the $ sign, followed by the name of the variable. The code snippet given below shows it.
+
+``` shell
+ <?php
+    $txt = "Hello world!";
+    $x = 6;
+    $y = 10.5;
+ ?>
+ ```
+ 
+Variable lifecycle: In PHP variables have a default value. If a variable is not declared before you attempt to use it, its value will be NULL. It is unset. So you can't use it by writing `isset($variable)` before using it.