Resolution
Probably it happens because PHP is unable to load dynamic library /usr/lib/php4/mysql.so due to the fact that libmysqlclient was updated after the installation.Also you might have seen the following failed dependencies message during mysql server installation:
error: Failed dependencies:
libmysqlclient.so.10 is needed by (installed) php-mysqlIn this case PHP itself needs to be recompiled as well.
Keywords: libmysqlclient