Question

I have 3 JSON strings coming in with POST and want to merge these into a 2 dimensional array and save in JSON format to the database.
For this example I have image URL’s, alt descriptions and booleans isfavorite

$url_arr = json_decode('["http://site.com/001.jpg","http://site.com/003.jpg","http://site.com/002.jpg"]');
$alt_arr = json_decode('["testing internat chars àèéìòóù stop","second description",""]'); // UTF-8 supported
$isFav_arr = json_decode('["true", "false", "false"]'); // strings need to be converted to booleans

// merge into 2 dimensional array
// $img_arr = array_merge($url_arr, $alt_arr, $isFav_arr); // doesn't work, just add's to the end
// ...

// save 2D JSON in database
$to_db = json_encode($img_arr);

Answer 1

Just string concatenate:

$to_db = '[' 
       . '["http://site.com/001.jpg","http://site.com/003.jpg","http://site.com/002.jpg"]'
       . ',["testing int chars àèéìòóù stop","second description",""]'
       . ',["true", "false", "false"]'
       . ']';

Unless you want to work with the values in the Json string, you dont need any en/decoding.
You can use http://www.jsonlint.com/ to validate it (removed jsonlint and print_r dumps to make some space)