从现有数组创建数组堆栈

Let's say I have an array like this:

$mail[0] = "test1@gmail.com";
$mail[1] = "test2@gmail.com";
$mail[2] = "test3@gmail.com";
$mail[3] = "test4@gmail.com";
$mail[4] = "test5@gmail.com";
$mail[5] = "test6@gmail.com";

Now I want to convert these mails to string and put them in array after every 3 mail of $mail separated by a comma. Something like this -

$email[0] = "test1@gmail.com, test2@gmail.com, test3@gmail.com";
$email[1] = "test4@gmail.com, test5@gmail.com, test6@gmail.com";
$email[2] = "test71@gmail.com, test8@gmail.com, test9@gmail.com";

How can I do this ?

Slight modification of your code:

for($i=0; $i<sizeof($oneusers); $i+=3){
    $new[] = $oneusers[$i].", ".$oneusers[$i + 1].", ".$oneusers[$i + 2];
}

print_r($new);

Going further you can add additional check whether $oneusers[key] exists.

Another code, which will do the same but without writing i, i+1, i+2:

$chunks = array_chunk($oneusers, 3);
foreach ($chunks as $chunk) {
    $new[] = implode(',', $chunk);
}

print_r($new);