MySQL won't start after upgrading to PHP 5.3
I think the problem stems from the fact that the upgrade was done trying to use the mysql schema folder of the previous version of MySQL, since the message "Can't open the mysql.plugin table" appears.

MySQL 5.1/5.5 uses a table called mysql.plugin

Prior versions (MySQL 5.1/4.x/3.x) do not use mysql.plugin.

I also noticed another oddity. The error message also said the highest file format is Barracida. The innodb_file_format of earlier versions of mysql is Antelope. You may want to export the InnoDB data in the previous version of mysql you had running. Otherwise, you may not have proper access to the InnoDB files.

Here is what you should do:

Step 1) Make a backup copy of that mysql folder. (/var/lib/mysql/mysql and /var/lib/mysql)

Step 2) You should reinstall the MySQL 5.0 version that was there before.

Step 3) Once mysql 5.0 is back up, perform a mysqldump of all databases except the mysql schema to /root/OldData.sql. Make sure you get triggers, stored procedures and views as follows:

For example, if you have 5 databases, (information_schema, db1, db2, db3, and mysql), do not include information_schema and mysql. Just mysqldump all the others.

mysqldump -hhostaddr -umyuserid -pmypassword --routines --triggers --databases db1 db2 db3 &gt; /root/OldData.sql


Step 4) Capture the mysql grants from MySQL 5.0 as follows:

mysql -hhostaddr -umyuserid -pmypassword --skip-column-names -A -e"SELECT CONCAT('SHOW GRANTS FOR ''',user,'''@''',host,''';') FROM mysql.user WHERE user&lt;&gt;''" | mysql -hhostaddr -umyuserid -pmypassword --skip-column-names -A | sed 's/$/;/g' &gt; /root/MySQLGrants.sql


Step 5) service mysql stop

Step 6) mv /etc/my.cnf /etc/my.cnf1

Step 7) Clear away /var/lib/mysql (Make sure you did Step 1)

Step 8) Uninstall mysql 5.0

Step 9) Install mysql 5.5 (MySQL 5.5 does not auto-install anymore)

Step 10) mv /etc/my.cnf1 /etc/my.cnf

Step 11) service mysql start

Step 12) Login to mysql and from the mysql client run the scripts

Step 13) Load the two SQL scripts

mysql&gt; source /root/MySQLGrants.sql
mysql&gt; source /root/OldData.sql


You should fine from here.

Give it a Try !!!