In this recent post to his blog Doug Brown spends a little time comparing (at a high-level) some of the differences between ASP and PHP.
The difference between PHP and ASP is that ASP is a Microsoft product based on visual basic syntax whereas PHP has C and Java based syntax. ASP works better on Microsoft servers.
He describes the target audience for each language and talks about the environments that they work best in as well as some general statements about their speed and flexibility. Basing his judgment on the facts he noted, he suggests PHP as the best alternative of the two for being more flexible, running in more places and being a bit faster overall.