Convert a flat, indexed array to a URL querystring

Your requested string will work as a querystring, but it is not a url-encoded querystring. I'll demonstrate solutions for both variations. Demo

For a fully url-encoded string which is effectively identical to your desited result, use native PHP function http_build_query(). This will encode the square braces and explicitly declare the indexes for each element of the array.

$array = ['393451234567', '393479876543'];

echo http_build_query(['recipients' => $array]);

recipients%5B0%5D=393451234567&recipients%5B1%5D=393479876543

Alternatively, achieve your desired result by prepending the key assignment string to each array value, then implode with &.

echo implode('&', substr_replace($array, 'recipients[]=', 0, 0));

recipients[]=393451234567&recipients[]=393479876543

Both snippets above will correctly return an empty string if given an empty array.

Another option:

foreach ($array as $key => $value){
    $array[$key] = (($key == 0) ? '' : '&').'recipients[]='.$value;
}
$result = implode('',$array);

A foreach loop allows you to conctenate your string. I include a check to avoid appending the & on the first part of the string.

Pointing this out as an option, but the other way is simpler!

Try this:

vsprintf('recipients[]=%s&recipients[]=%s', $recip);

Another option

foreach ($recip as $ip){
    $array[] = 'recipients[]=' . $ip;
}
$result = implode('&',$array);