If you’re ever in a situation where something is only happening intermittently, and only on a live server, and only while it’s under load… Lets say its not generating any error_log or stderr output, and you cant run it manually to reproduce… (we’ve all been in this situation) How do you get any debugging output at all?

Step 1: add this to the top of your entry point php file

if ( $_SERVER['REMOTE_ADDR'] == '127.0.0.1' ) {
       error_log( ' :: ' . getmypid() );
       sleep( 10 );
}

Step 2: use curl on the localhost to make the request

Step 3: (this assumes your error log is /tmp/php-error-output) run the following command in a second (root) terminal window

strace -p $(tail -n 1000 /tmp/php-error-output | grep ' :: ' | tail -n 1 | sed -r s/'^.+ :: '//g) -s 10240 2>&1

Good luck…