Courses

<?php

$coursesStudents = [];
$input = readline();

while ($input !== 'end') {
    $cmd = explode(' : ', $input);
    $courseName = $cmd[0];
    $studentName = $cmd[1];

    if (!key_exists($courseName, $coursesStudents)) {
        $coursesStudents[$courseName] = [];
    }
    $coursesStudents[$courseName][] = $studentName;

    $input = readline();
}

uksort($coursesStudents, function ($key1, $key2) use ($coursesStudents) {
    $countCourses1 = count($coursesStudents[$key1]);
    $countCourses2 = count($coursesStudents[$key2]);
    return $countCourses2 <=> $countCourses1;
});

foreach ($coursesStudents as $course => $students) {
    asort($coursesStudents[$course]);
    echo $course . ": " . count($coursesStudents[$course]) . PHP_EOL;
    foreach ($coursesStudents[$course] as $student) {
        echo "-- $student" . PHP_EOL;
    }
}