mysql_real_escape_string is undefined

Update as mentioned in comment, mysql_ has been deprecated since 5.5:

The mysql extension has been deprecated since PHP 5.5. The mysqli or PDO extension should be used instead. The deprecation has been decided in mysql_deprecation, where a discussion of the reasons behind this decision can be found.

and removed in PHP 7.

mysql_real_escape_string() is standard part of MySQL function "batch" and should always work if the extension is loaded correctly.

Does any another mysql_ function work? (It should not)

Make sure, that you have this line uncommented in your php.ini:

extension=mysql.so

Also it'd be wise to use mysqli or PDO instead (mysql_ is deprecated), they both can take care of escaping for you.

In my case I used mysqli_real_escape_string instead of mysql_real_escape_string.

Interestingly, after exploring all the other solutions here, I realized the problem is actually due to the php5-mysql extension not having been installed yet - it's not installed by default on a fresh Ubuntu, neither when u install fresh php. So, for me the solution became: install the php5-mysql extension:

sudo apt-get install php5-mysql

After this, I wasn't getting those nasty mysql_* errors again ;-)