$arr = [
['title' => 'test_title1', 'descr' => 'test_desc1', 'img'=>'test_img1'],
['title' => 'test_title2', 'descr' => 'test_desc2', 'img'=>'test_img2'],
['title' => 'test_title3', 'descr' => 'test_desc3', 'img'=>'test_img3'],
];
$json = [];
foreach ($arr as $value) {
$json[] = [
'title_bl' => $value['title'],
'descr_bl' => $value['descr'],
'img_bl' => $value['img']
];
}
$json = json_encode($json);
print_r($json);
Result:
[{"title_bl":"test_title1","descr_bl":"test_desc1","img_bl":"test_img1"},{"title_bl":"test_title2","descr_bl":"test_desc2","img_bl":"test_img2"},{"title_bl":"test_title3","descr_bl":"test_desc3","img_bl":"test_img3"}]
Так же, как и с одномерными.