php - How to prevent duplicate email addresses from being registeres? -

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i have been making login/register system , 1 problem have run not allowing duplicate email addresses being registered. want work database wont accept data duplicate email , user alerted too. sort of new php unsure of how this. thanks.

my php

if (empty($_post['email'])) {     $error[] = 'please enter email '; } else {      if (preg_match("/^([a-za-z0-9])+([a-za-z0-9\._-])*@([a-za-z0-9_-])+([a-za-z0-9\._-   ]+)+$/", $_post['email'])) {        //regular expression email validation         $email = $_post['email'];     } else {          $error[] = 'your email address invalid  ';     }   }   if (empty($_post['password'])) {     $error[] = 'please enter password '; } else {     $password = $_post['password']; }   if (empty($error)) //send database if there's no error '  { // if everything's ok...      // make sure email address available:     $query_verify_email = "select * members  email ='$email'";     $result_verify_email = mysqli_query($dbc, $query_verify_email);     if (!$result_verify_email) {//if query failed ,similar if($result_verify_email==false)         echo ' database error occured ';     }      if (mysqli_num_rows($result_verify_email) == 0) { // if no previous user using email .           // create unique  activation code:         $activation = md5(uniqid(rand(), true));           $query_insert_user = "insert `members` ( `username`, `email`, `password`, `activation`) values ( '$username', '$email', '$password', '$activation')";           $result_insert_user = mysqli_query($dbc, $query_insert_user);         if (!$result_insert_user) {             echo 'query failed ';         }          if (mysqli_affected_rows($dbc) == 1) { //if insert query successfull.   mysqli_close($dbc);//close db connection   } // end of main submit conditional.  ?>

the html

<form action="./index.php#openmodal2" method="post" class="registration_form">   <fieldset> <legend>registration form </legend>  <p>create new account</p>  <div class="elements">   <label for="username">name :</label>   <input type="text" id="username" name="username" size="25" /> </div> <div class="elements">   <label for="email">e-mail :</label>   <input type="text" id="email" name="email" size="25" /> </div> <div class="elements">   <label for="password">password:</label>   <input type="password" id="password" name="password" size="25" /> </div> <div class="submit">  <input type="hidden" name="formsubmitted" value="true" />   <input type="submit" value="register" /> </div>

add unique constraint on email column in table members:

alter table members add unique (email);

typically, when create table rather altering table afterwards.


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