Debugging is the process of examining your application for errors. The process of debugging is accomplished by setting breakpoints and watches in your code and running it in the debugger. This enables you to execute your code one line at a time or breakpoint-to-breakpoint and examine the state of your application in order to discover any problems.
When you start a PHP debugging session, a browser window opens. By default the application stops at the first line of the program. You must switch back to the IDE to continue the debugging session. You switch back and forth between the IDE and your browser throughout your debugging session.
Additional debugger windows also appear automatically at the bottom of your screen. These windows help you keep track of breakpoints, variables, sources, the call stack, and threads.
You can also debug applications that are running on a remote machine by setting up path mapping to a custom URL.
For more details see the following document.