How to add specific key to array values?

I stored links to $fotolar variable and I converted to array with explode.

My code:

$ex = explode('<br>', $fotolar);

echo "<pre>";
print_r($ex);
echo "</pre>";

Output:

Array
(
    [0] => 
    https://www.mersevkids.com/uploads/images/202112/img_1920x_61b1f9e4ed8c48-88505152-13660630.jpeg
    [1] => 
    https://www.mersevkids.com/uploads/images/202112/img_1920x_61b16a438951a1-13859326-45341947.jpg
    [2] => 
    https://www.mersevkids.com/uploads/images/202112/img_1920x_61b16a3f2df271-08655153-10027442.jpg
)

How can I add "url" key to this array?

For example:

    Array
    (
        [0] => Array
(
           [url] =>
        https://www.mersevkids.com/uploads/images/202112/img_1920x_61b1f9e4ed8c48-88505152-13660630.jpeg
)
        [1] => Array
(
           [url] =>
        https://www.mersevkids.com/uploads/images/202112/img_1920x_61b16a438951a1-13859326-45341947.jpg
)
        [2] => Array
(
           [url] =>
        https://www.mersevkids.com/uploads/images/202112/img_1920x_61b16a3f2df271-08655153-10027442.jpg
)
    )

>Solution :

After exploded string to array, you can loop through this array to create a new array like this:

<?php

$str = <<<STR
https://www.mersevkids.com/uploads/images/202112/img_1920x_61b1f9e4ed8c48-88505152-13660630.jpeg<br>
https://www.mersevkids.com/uploads/images/202112/img_1920x_61b16a438951a1-13859326-45341947.jpg<br>
https://www.mersevkids.com/uploads/images/202112/img_1920x_61b16a3f2df271-08655153-10027442.jpg
STR;

$arr = explode('<br>', $str);
$result = [];

foreach ($arr as $v) {
    $result[]['url'] =  $v;
}

var_dump($result);