1. Why is this happening?
The entire ext/mysql PHP extension, which provides all functions named with the prefix mysql_, is officially deprecated as of PHP v5.5.0 and will be removed in the future.
It was originally introduced in PHP v2.0 for MySQL v3.23, and no new features have been added since 2006. Coupled with the lack of new features are difficulties in maintaining such old code amidst complex security vulnerabilities.
The manual has contained warnings against its use in new code since June 2011.
As the error message suggests, there are two other MySQL extensions that you can consider: MySQLi and PDO_MySQL, either of which can be used instead of ext/mysql.
They differ slightly, but offer a number of advantages over the old extension including API support for transactions, stored procedures and prepared statements (thereby providing the best way to defeat SQL injection attacks). Ulf Wendel has written a thorough comparison of the features.
Hashphp.org has an excellent tutorial on migrating from ext/mysql to PDO.
3. I understand that it's possible to suppress deprecation errors by setting error_reporting in php.ini to exclude E_DEPRECATED:
error_reporting = E_ALL ^ E_DEPRECATED
What will happen if I do that?
Yes, it is possible to suppress such error messages and continue using the old ext/mysql extension for the time being. But you really shouldn't do this—this is a final warning from the developers that the extension may not be bundled with future versions of PHP. Instead, you should take this opportunity to migrate your application now, before it's too late.
Note also that this technique will suppress all E_DEPRECATED messages, not just those to do with the ext/mysql extension: therefore you may be unaware of other upcoming changes to PHP that may affect your application code.
Found this somewhere else, hope it helps
