When display_errors is set to off in the php.ini file and I turn it back on with ini_set. it displays fatal errors like missing functions, but not parse errors like missing quotes.

from php.ini file:
error_reporting  =  E_ALL & ~E_NOTICE
display_errors = Off
display_startup_errors = Off
log_errors = On
error_log = "/tmp/php.log"

Test Script:
<?php
ini_set('display_errors',1);

# echo "dfadf"asdf;
# missingfunction();

phpinfo(4);
?>

Run the script as is and the variables in phpinfo all look a-ok, display_errors is set to "On" locally. 

Uncomment the echo line and it logs the error to the file but does not display the error. The kind of error is a parse error.

Now try the missingfunciton line,  it logs the error to the file and displays the error to the screen, the desired action. The kind of error is a fatal error.

Confiure for my PHP is:
--with-apache=xxx
--with-mysql \
--enable-inline-optimization \
--with-pspell \
--with-gd \
--with-jpeg \
--with-jpeg-dir=/usr/lib \
--with-png \
--with-png-dir=/usr/lib \
--with-zlib