how to deal with this (hack) attack?

Hi there, I am running a medium size php/mysql website. In the last days I recieved more and more mysql errors due to url tempering. Now my website went down for at least 5 minutes, or more detailed the database server mysql 3.x went down. In the apache logs I found requests like "connection:close" comming from unspecified clients. The site is running again (I did not do anything). Can anybody give me a hint on how to prevent such atacks? What is that connection:close thing? Any help is greatly appreciated! Merlin