Multiple file upload with optional items - php

Question

I have a form with multiple inputs like this:
<div id="users">
<input type="text" name="title[]" required>
<input type="file" name="images[]">
<input type="text" name="title[]" required>
<input type="file" name="images[]">
<input type="text" name="title[]" required>
<input type="file" name="images[]">
</div>
<input type="button" value="add new user">
<input type="submit" value="submit">
the problem is that selecting image is optional and if the user doesn't select any image for any of titles the posted data would be like this:
title = ["title1","title2","title3"]
images = [image1.jpg,image3.jpg]
the count of users is not fixed and user can add any desired number of titles and images by clicking on the add new user button. so "add new user" button will add a pair of these inputs to users div:
<input type="text" name="title[]" required>
<input type="file" name="images[]">
is there any way to detect if the file is selected or not? or to send a default value for not selected file inputs? I want to set null value for image if no file is selected, like this:
user1:{title="title1",image=image1.jpg}
user2:{title="title2",image=null}
user3:{title="title3",image=image3.jpg}
.
.
.

Answer 1

You can check if particular index is set for file:
var_dump($_POST['title'][0]);
if (isset($_FILES['images']['tmp_name'][0])) {
var_dump($_FILES['images']['tmp_name'][0]);
}

Answer 2

All you have to do is process the 2 arrays and check there is a matching image with each title. If there is not a image for each title, put anything that you like in place of the image name
<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
$output = [];
foreach( $_POST['title'] as $idx => $title){
$u = $idx+1;
$output['user'.$u] = ['title'=>$title,
'images'=> $_POST['images'][$idx] == '' ? 'NULL' : $_POST['images'][$idx]];
}
$j = json_encode($output);
echo $j;
}
RESULT
{
"user1":{"title":"asss","images":"NULL"},
"user2":{"title":"dd","images":"hammer.jpg"}
}

Answer 3

Thank you everybody, Pavol Velky was right partially, Although the array size is less than the number of inputs, but the browser will set an index to the array, in example if you have two inputs and just select the second one, the array will be like : item:[1:{}] (if you select first one: item:[{}]) and you can check the order by the index number.