Encoding a single key in associative array to base64 - php

Question

I am trying to retrieve user data from a SQL table where one of the columns is blob_medium. I run the SQL query properly and get the data in the php script.
if(mysqli_num_rows($result)){
while($row=mysqli_fetch_assoc($result)){
$result_array['user_data'][]=$row;
}
}
Now, to json_encode this data, I need to encode the data of user_pic column to base 64. For that I am trying this. But, it seems I am doing something wrong. Any kind of help would be appreciated.
foreach($result_array as $key){
foreach($key as $key2){
//print_r(base64_encode($key2['user_pic']).'<br/>'.'<br/>');
$key2['user_pic'] = base64_encode($key['user_pic']);
//print_r(($key['user_pic']).'<br/>'.'<br/>');
}
}
When I uncomment the print_r statements my data is printed in base64 format but the data of the assoc array is not changing.

Answer 1

That's because the array's $key and $keys in the for loop are copies. If you want them to modify the original you can either do it by specifying them to be references, not copies:
foreach($result_array['user_data'] as &$key){
$key['user_pic'] = base64_encode($key['user_pic']);
}
Or by explicit index into the original:
foreach($result_array['user_data'] as $index => $key){
$result_array['user_data'][$index] ['user_pic'] = base64_encode($key['user_pic']);
}

Answer 2

That's because you're changing the $key2 array. A temporary value created by your foreachloop. I myself would recommend using a for loop in this specific situation, because I was told to never use a loop within a loop if I can prevent it, and it makes things a lot easier to read:
for($i=0; $i < count($result_array['user_data']); $i++){
$encodedUserPic = base64_encode($result_array['user_data'][$i]['user_pic']);
$result_array['user_data'][$i]['user_pic'] = $encodedUserPic;
}