<?php

$my_array = array(
    "adad" => "value X",
    "yy" => "value Y",
    "hkghgk" => "value Z",
    "pp" => "value ZZ",
);

$start_key = 'yy';
foreach(array_slice(
    $my_array,
    array_search(
        $start_key,array_keys($my_array)
    ),
    null,
    true) as $key => $value) {
    echo $key , ' => ', $value, PHP_EOL;
}