$arr1 = array (1, 4, 5, 7, 9, 15, 18, 20 );
print_r ( input_array ( $arr1, 5, 6 ) );

function input_array($array, $value, $input) {
    $key = array_search ( $value, $array );
    if ($key !== false) {
        array_splice ( $array, $key, 1, array ($array [$key], $input ) );
        return $array;
    }
    return false;
}