Add some PHP code to check for max enrollment in Moodle

I am looking to modify the enrol_paypal plugin to offer a way to limit the number of students per course that can enroll that way. I have figured out a short way to do it, but am not good enough in PHP to make it work. From [login to view URL] in the enrol_paypal folder: function enrol_page_hook(stdClass $instance) { global $CFG, $USER, $OUTPUT, $PAGE, $DB; ob_start(); if ($DB->record_exists('user_enrolments', array('userid'=>$USER->id, 'enrolid'=>$instance->id))) { return ob_get_clean(); } if ($instance->enrolstartdate != 0 && $instance->enrolstartdate > time()) { return ob_get_clean(); } if ($instance->enrolenddate != 0 && $instance->enrolenddate < time()) { return ob_get_clean(); } $course = $DB->get_record('course', array('id'=>$instance->courseid)); $context = context_course::instance($course->id); $shortname = format_string($course->shortname, true, array('context' => $context)); $strloginto = get_string("loginto", "", $shortname); $strcourses = get_string("courses"); // Pass $view=true to filter hidden caps if the user cannot see them if ($users = get_users_by_capability($context, 'moodle/course:update', 'u.*', '[login to view URL] ASC', '', '', '', '', false, true)) { $users = sort_by_roleassignment_authority($users, $context); $teacher = array_shift($users); } else { $teacher = false; } if ( (float) $instance->cost <= 0 ) { $cost = (float) $this->get_config('cost'); } else { $cost = (float) $instance->cost; } if (abs($cost) < 0.01) { // no cost, other enrolment methods (instances) should be used echo '<p>'.get_string('nocost', 'enrol_paypal').'</p>'; } else { /*REQUESTED FUNCTIONALITY: This is where we can put the check for maximum enrolled students: If ((count(userid) from mdl_enrol_paypal where payment_status = 'Completed' and courseid = $instance->courseid) > 75) {print "Course is full.";} else{*/ // Calculate localised and "." cost, make sure we send PayPal the same value, // please note PayPal expects amount with 2 decimal places and "." separator. $localisedcost = format_float($cost, 2, true); $cost = format_float($cost, 2, false); ... I do not need this to appear in any settings page right away, but may want to add that functionality later.