How can I get MAMP to tell me what went wrong with php code?

Just as you reported, you must have display_errors set to on. This can be done either by changing the conf file or by using .htaccess like so:

<IfModule mod_php5.c>
    php_flag display_errors on
</IfModule>

Additionally, you can do this with ini_set() like so:

ini_set('display_errors', 1);

One last thing, you can also check /Applications/MAMP/logs which has three different error log files.

Try opening terminal and run this command:

tail -f /Applications/MAMP/logs/php_error.log

When you want to stop "following" (the -f switch) the log file, just type control+C.

You can also access MAMP errors using the Mac "Console" app to read the php_error.log file.

I find this easiest to access by using spotlight and typing in "error.log".

( it won't find it if you type "php_error.log", you must type "error.log" )

It looks like this :