<?php

$arr = [
  "user_id" => 1,
  "description" => [
    "текст первый",
    "текст второй",
    "и даже третий"
  ],
  "title" => []
];


$new_arr = [];
foreach ($arr['description'] as $desc) {
	$tmp = $arr;
	$tmp['description'] = $desc;
	$new_arr[] = $tmp;
}

print_r($new_arr);