PHP & MySQL Different users seeing specific information

Program:

Program that has different level of User Permissions, through a login script. Allowing each user to read there specific account data.

using a system of PHP/Mysql presumably.

Question:

I can read / write data using certain examples, but just modifying for my need. The problem i run into is, how do I get the database to only pull THAT users specific data? Any ideas, or scripts would be helpful.

Thank you everyone