Колега, на 7-задача просто го направи с implode и ще ти даде 100/100.

<?php
        if(isset($_GET['key-value-pairs']) && isset($_GET['delimiter']) && isset($_GET['target-key']))
        {
            // get the data
            $input = $_GET['key-value-pairs'];
            $delimiter = $_GET['delimiter'];
            $targetKey = $_GET['target-key'];

            // split and trim the array
            $arr = explode("\n", $input);
            $arr = array_map('trim', $arr);

            $arrResult = [[]];

            for($i = 0; $i < count($arr); $i++)
            {
                $line = explode($delimiter, $arr[$i]); // explode the array
                $key = $line[0];
                $value = $line[1]; // save the key and the value

                if(isset($arrResult[$key]))
                    array_push($arrResult[$key], $value);

                else
                {
                    $arrResult[$key] = [];
                    array_push($arrResult[$key], $value);
                }

            }

            if(isset($arrResult[$targetKey]))
                echo implode("<br>", $arrResult[$targetKey]);

            else
                echo "None";
        }
    ?>