How to check for a php-extension?

Hi, i wrote a script, which uses the IMAP-extension. But if s.o. uses it on a machine, which doesn't support IMAP functions, i get an error message. How can i prevent this error: Fatal error: Call to undefined function: imap_open() in /home/test/public_html/test/imap_test.php on line 8 and replace it with s.th. like: "This function doesn't work, due to missing IMAP extension". I already tried some stuff, but the problem is, that php exits the script as soon as it hits the first IMAP related function. Thanks for your help, Duncan