在AJAX中使用php文件的最佳实践

I'm beginning web development with php and am testing jQuery ajax calls to a php file to run functions. But I've noticed that the php file loads into the resources each time I call it with an AJAX POST method. What is the best solution to prevent this occurrence? Also, are there better coding practices to use when performing multiple function calls (what I'm used to calling web services or web methods in the c# world) from a single file in php?I'm beginning web development with php and am t