Xdebug can only listen on one running PHP process at a time. There are usually two possibilities for this.
1) As @adrianGW says, there may be another process already connected to the debugger.
2) Your program is trying to load another PHP process, and this process cannot start until xdebug releases the current thread. It is well known that PHPunit will run tests in its threads so that they do not spoil each other. Or you make an http request in your php script application on the same server, and this second request is waiting for the first to complete so that you are blocked until the first script ou times
There are two errors in PHPstorm:
1) You can change Max Simultaneous Conections to a number greater than 1
2) You can enable Ignore external connections through unregistered servers configurations , but this will only work if the reason for the additional flows is something like a request to another domain on the same server, which xdebug can distinguish as an unregistered server.
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