JSON Database update loosing key values - php

Question

I got this in my DB (23 is my picture ID):
{"23" : {"13x18":"5","20X30":"5","30x45":"4","digital":"4"}}
I want to add another data just like this one in the same spot so it would look like this :
["23" : {"13x18":"5","20X30":"5","30x45":"4","digital":"4"},
"42" : {"13x18":"2","20X30":"1","30x45":"3","digital":"1"}]
Here is the code is use :
$clientChoice = [];
$clientChoice[$post['picId']] = $test;
$select = (new GalleryCategoriesModel)->findByUrl($post['url']);
if($select['clientChoice'] != null){
$tabs = json_decode($select['clientChoice'], true);
$new = array_merge($tabs, $clientChoice);
$chosen = json_encode($new);
} else {
$chosen = json_encode($clientChoice);
}
$update = (new GalleryCategoriesModel)->update(['clientChoice'=>$chosen], $post['id']);
And here is the result i got with it :
[{"13x18":"5","20X30":"5","30x45":"4","digital":"4"},
{"13x18":"2","20X30":"1","30x45":"3","digital":"1"}]
What am I missing or doing wrong?
----------------- CORRECTION ------------------
Thanks to the answer given, here is what i do now :
$clientChoice = [];
$clientChoice[$post['picId']] = $test;
$select = (new GalleryCategoriesModel)->findByUrl($post['url']);
if($select['clientChoice'] != null){
$tabs = json_decode($select['clientChoice'], true);
$new = $tabs + $clientChoice;
$chosen = json_encode($new);
} else {
$chosen = json_encode($clientChoice);
}
$update = (new GalleryCategoriesModel)->update(['clientChoice'=>$chosen], $post['id']);
And here is what I got in my DB :
{"25":{"13x18":"1","20X30":"3","30x45":"5","digital":"1"},
"37":{"13x18":"4","20X30":"8","30x45":"3","digital":"2"}}

Answer 1

Your issue is with your call to array_merge. From the manual:
Values in the input arrays with numeric keys will be renumbered with incrementing keys starting from zero in the result array
So your two arrays that have keys of 23 and 42 end up merged as:
Array
(
[0] => Array
(
[13x18] => 5
[20X30] => 5
[30x45] => 4
[digital] => 4
)
[1] => Array
(
[13x18] => 2
[20X30] => 1
[30x45] => 3
[digital] => 1
)
)
You can work around this by using the array union operator (+), which will preserve the keys from both arrays (although it will ignore $clientChoice if the key is the same as one already existing in $tabs), however since they are id values I assume this should not occur.
$new = $tabs + $clientChoice;
In this case, $new contains:
Array
(
[23] => Array
(
[13x18] => 5
[20X30] => 5
[30x45] => 4
[digital] => 4
)
[42] => Array
(
[13x18] => 2
[20X30] => 1
[30x45] => 3
[digital] => 1
)
)
Demo on 3v4l.org