Here is my solution:

<?php
 
$frogs = explode(' ', readline());
$input = readline();
 
while ($input !== 'Print') {
 
    $args = explode(' ', $input);
 
    switch ($args[0]) {
        //Join {name}":
        //A frog comes on the riverside and you need to add it in the end of your collection. Frog names will never repeat.
        case 'Join':
            $name = $args[1];
            if (!in_array($name, $frogs)) {
                $frogs[] = $name;
            }
            break;
        //Jump {name} {index}"
        //A frog jumps out of the water and joins the other frogs. You need to add it in your collection on the given index, if the index exists.
        case 'Jump':
            $name = $args[1];
            $index = $args[2];
            if (key_exists($index, $frogs)) {
                array_splice($frogs, $index, 0, $name);
            }
            break;
        //Dive {index}":
        //oThe frog on the given index has decided to jump into the water. You have to remove it from your collection, if the index exists.
        case 'Dive':
            $index = $args[1];
            if (key_exists($index, $frogs)) {
                array_splice($frogs, $index, 1);
            }
            //var_dump($frogs);
            break;
        //First/Last {count}":
        //oPrint the first/last {count} frogs separated by a single space. If the count requested is more than the frogs- just print them to the end.
        //"{frog} {frog} {frog}"
        case 'First':
            $count = $args[1];
            if ($count > count($frogs)) {
                echo implode(' ', $frogs) . PHP_EOL;
            } else {
                echo implode(' ', array_slice($frogs, 0, $count)) . PHP_EOL;
            }
            break;
        case 'Last':
            $count = $args[1];
            if ($count > count($frogs)) {
                echo implode(' ', $frogs) . PHP_EOL;
            } else {
                echo implode(' ', array_slice($frogs, count($frogs) - $count, $count)) . PHP_EOL;
            }
            break;
        //Print Normal/Reversed"
        //oPrint the names of the frogs in your collection in normal (in the order they have been added) or reversed order in the format described below, then stop the program:
        //"Frogs: {frog1} {frog2}… {frogn}"
        case 'Print':
            $print = $args[1];
            if ($print == 'Normal') {
                echo 'Frogs: ' . implode(' ', $frogs);
            }
            if ($print == 'Reversed') {
                echo 'Frogs: ' . implode(' ', array_reverse($frogs));
            }
            exit();
            break;
    }
    $input = readline();
}