在txt文件中搜索变量并检查PHP是否存在

Question

I have a coupon page where the user enters their email address and gets a random string to their email.

I want to check first if the user has already signed up for a coupon code by searching in coupon.txt; if one is already present an error is output, otherwise a coupon is sent.

This is my code for sending the email:

$mailed = false;
if (!empty($name) && !empty($email) )
    $mailed = mail($emailTo, $subject, $body, $headers, $randomString);

if($mailed == true) {
    echo json_encode(array('status' => 'ok')); 
    file_put_contents("coupon.txt", $email. . $RandomString .  PHP_EOL, FILE_APPEND);
    mail($email, "praia", $message , $headers2);
} else {
    echo json_encode(array('status' => 'error')); 
}