dynamic code

imagine having a database to which chunks of php code (among plain text) are submitted. how can i actually read this code from the database and then execute it within one script? the main problem is that i want to output plain text/html mixed with php. say, i have a field in my database containing html docs that can optionally contain php code. in this case, eval($body) produces parsing errors. do i have to parse them myself first and alternately eval() the php parts? tia, sam