PHP mail() force single recipient - php

Question

What is the best way to force mail() to send only to first recipient, even if user provides multiple recipients in $email... I need to filter user input somehow...?
$email = '1#mail.com, 2#mail.com, 3#mail.com...';
mail( $email, $subject, $body, $extra ) //send to: 1#mail.com

Answer 1

You may write:
$emails = explode(',',$email);
Now $emails[0] is the first email you can use in the mail() function.

Answer 2

You could explode the string by comma and only throw the first element of the resulting array in mail().
$email = '1#mail.com, 2#mail.com, 3#mail.com...';
$recips = explode(",", $email);
if(filter_var(trim($recips[0]), FILTER_VALIDATE_EMAIL) !== false) {
mail(trim($recips[0]), $subject, $body, $extra ) //send to: 1#mail.com
} else {
die("no valid email");
}

Answer 3

Imagine the user does not separate the adresses by using a comma. then this will not work and just trim()ing the adress will also not prevent you from giving multiple emails to mail().
You will somehow also need to check the adress to prevent syntax errors, f.e. using filter_var():
$emails = explode(',', $email);
$email = filter_var(trim($emails[0]), FILTER_VALIDATE_EMAIL);
if ($email === false) {
exit();
}
some more sophisticated checking can also be done using regular expressions:
$regex = "/^[a-zA-Z\d][\w\.-]*[a-zA-Z\d]#[a-zA-Z\d][\w\.-]*\.[a-zA-Z]{2,}$/i";
$emailString = '1#mail.com, 2#mail.com, 3#mail.com...';
$emailArray = explode(',', $emailString);
$isValdid = preg_match($regex, $emails[0]);
if ($isValid === 0) { // 1 for match, 0 for mismatch, false for error
echo "please supply a valid adress.";
}