I just starting out learning PHP, I\'ve been developing web apps in ASP.Net for a long time. I was wondering if there are any PHP specific security mistakes that I should be
Language Vs Programmer. You can write the most serious vulnerability and you won't get a warning or error message. Vulnerabilities can be as simple as adding or removing 2 characters in your code. There are hundreds of different types of vulnerabilities that affect PHP applications. Most people think of XSS and Sql Injection because they are the most popular.
Read the OWASP top 10.