The previous answer was pretty good, let me expand on that. ASP.net is also license software…ie, you have to pay to use it. PHP is Open Source, which means that rather than being developed by Microsoft, it is developed by the community.
Typically Open Source software has a lower cost of ownership because you potentially save fees on Operating System software (if you use Linux), development tools, and related software (like your database software).
PHP is designed to work natively with MySQL, which means it offers a lot of functions built-in to make accessing your database easier. Also, PHP has other extensions to help make it even more useful (for instance, PEAR).
I hope that helps.
Angela
In reply to the post after mine, ASP.net is not more powerful. There are very large companies currently using PHP (Capital One, for instance). MySQL, particularly since the release of 5.0, has many features that put it on a par with MS SQL Server and Oracle, including transaction based processing and stored procedures.
Most small/medium ecommerce sites are run on shared hosting servers. The best value for your money is typically on a Linux server running PHP/MySQL. If you need web hosting, check out Dreamhost (http://www.dreamhost.com). I’ve been using them for three years now. Enter elvenarch@yahoo.com as the email address of who referred you please.

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