Processing PHP within a string variable

On a project I am working on, I have a string variable, and inside the string is a mixture of PHP code and HTML. I want to be able to output the string with the PHP code inside the string processed. Right now the PHP isn't processed on output, it is merely cut out. I have been able to do this by writing the contents of the string to file and then including the file, however this is not a viable method for what I'm working on here. I was wondering if there is any way to output and process PHP code that is contained within a string? Thanks.