Database connection error (2): Could not connect to MySQL after PHP upgrade

[pawan]

I have upgraded my php from 5.3 to 5.4 and now I am getting frequent mysql error:
Database connection error (2): Could not connect to MySQL

is it related to php upgrade or some other reason.

I am also getting this error when I trying to log on ISPCONFIG admin

DB::__construct Too many connectionsDB::__construct Too many connectionsDB::query(SELECT * FROM sys_session WHERE session_id = '') -> mysqli_query Too many connections#0 db

ISPCONFIG SERVER LOG

PHP Warning: mysqli::close(): Couldn't fetch db in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 70
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
PHP Warning: mysqli::mysqli(): (08004/1040): Too many connections in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 61
DB::__construct Too many connections
PHP Warning: mysqli::query(): Couldn't fetch db in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 65
PHP Warning: mysqli::query(): Couldn't fetch db in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 66
PHP Warning: mysqli::ping(): Couldn't fetch db in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 106
PHP Warning: mysqli::query(): Couldn't fetch db in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 107
PHP Warning: mysqli::escape_string(): Couldn't fetch db in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 166
PHP Warning: mysqli::ping(): Couldn't fetch db in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 106
PHP Warning: mysqli::query(): Couldn't fetch db in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 107
PHP Warning: mysqli::close(): Couldn't fetch db in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 70
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
Failed loading /usr/lib/php5/20090626/xcache.so: /usr/lib/php5/20090626/xcache.so: cannot open shared object file: No such file or directory
PHP Warning: mysqli::mysqli(): (HY000/2002): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2) in /usr/local/ispconfig/server/lib/classes/db_mysql.inc.php on line 61
DB::__construct Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)

I am also getting this error repeatadely in ispconfig log

ERROR - Plugins directory missing: /usr/local/ispconfig/server/plugins-core/