[php-src] Issue #10721: Improved Bug Report for "header already sent" warning

Issue: https://github.com/php/php-src/issues/10721 Author: ianwhiteley WARNING RECEIVED PHP Warning: session_start(): Cannot start session when headers already sent in /filepath on line xx CURRENT SCENARIO & ISSUE This warning gives the line number xx of the session_start(). It does not tell you WHAT triggered the sending of headers before this point was reached, and where that point is. REQUESTED IMPROVEMENT Add the file / line number for WHEN the last headers were previously sent - as this can be very difficult to pinpoint.