Where is the php_error.log file in Acquia Dev Desktop?
Just checked in my installation of DevDesktop and found that my PHP logs are located in /Applications/DevDesktop/apache/logs/error_log. 

Although it says Apache logs, PHP errors get logged there as well.

You can find the log locations via 'Preferences' => 'Logs'. DevDesktop version 2 (Apr 13, 2017).