Warning: mysql_connect(): No such file or directory

Question

What are the technical reasons for why one shouldn't use mysql_* functions? (e.g. mysql_query(), mysql_connect() or mysql_real_escape_string())?

Why should I use something else even if they work on my site?

If they don't work on my site, why do I get errors like.

Answer 1

The MySQL extension:

Is not under active developments.

Is officially deprecated as of PHP 5.5 (released June 2013).
Has been removed entirely as of PHP 7.0 (released December 2015)

This means that as of 31 Dec 2018 it does not exist in any supported version of PHP. If you are using a version of PHP which supports it, you are using a version which doesn't get security problems fixed.
Doesn't support: Non-blocking, asynchronous queries, Prepared statements or parameterized queries, Stored procedures, Multiple Statements, Transactions.
The "new" password authentication method (on by default in MySQL 5.6; required in 5.7)
Any of the new functionality in MySQL 5.1 or later
Since it is deprecated, using it makes your code less future proof.

Lack of support for prepared statements is particularly important as they provide a clearer, less error-prone method of escaping and quoting external data than manually escaping it with a separate function call.