PHP - Log stacktrace for warnings?

There is an example of using set_error_handler() in conjunction with ErrorException to do just this:

https://php.net/manual/en/class.errorexception.php

You would just need to implement your custom logging functionality inside of the handler function.

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UPDATE:

Note, this also works for warnings, and many other error types. For full compatibility, see the manual for set_error_handler():

https://php.net/set_error_handler

Just throw this at the start of your script:

set_error_handler(function($severity, $message, $file, $line) {
    if (error_reporting() & $severity) {
        throw new ErrorException($message, 0, $severity, $file, $line);
    }
});

Remove the if you want to log everything, even if it's suppressed.