Added code block open and close around Mysqli example. (#26354)

* Update index.md

Added code block open and close around Mysqli example.

* Changed code block to use backticks.

* Removed php tags from code example

Removed php tags from code example.

* Added formatting, syntax highlighting

guide/english/php/working-with-databases/index.md

@@ -16,7 +16,7 @@ database simple and easy.
 
 Some Examples from Mysqli are-
 
-<?php
+```php
 $con=mysqli_connect("localhost","root","","db_name") or die("Invalid User or Password...cannot connect");
-?>
-here we are connecting to a database on the phpmyadmin structure with no password and database name-db_name
+```
+Here we are connecting to a database on the phpmyadmin structure, with no password, and database named `db_name`.