PHP Max execution time

I am running a script (called through a browser) that checks entries in a table one at a time. As the table has grown, the script takes longer to run than is allowed - 30 seconds. Two questions: 1) I am on a virtual host so I'm not sure I have access to php.ini, but can I change this using .htaccess? Do I need any special permission for this? 2) Is there a way to change this just for one script, i.e. I did find the set_time_limit() function but can I use something like set_time_limit(300) or does it have to be less than 30? Thanks in advance! -Lisi