Variable de variable indéfinie en php

//some times it's happend beacuse you did not define any variable at first of your code 
//for example
//$example = $php;
//here is no variable as ($php) for execution sowe need to define a variable call ($php = 25;)
//now error can be solved
//sometime you want to post somthing  like
$email = $_post['email'];
//but you get error as undefine varaible $email, cause of these sort of errors is 
//in php 8.0.0 you hgave to define a variable like this 
$email = isset($_post['email'])
//full code down below
<?php 
if(isset($_POST['sub'])){
      
      $email = $_POST['email'];
      // reading mysql in this part
      $result=  $conn->prepare('SELECT signup FROM email=?') ; 
      $result->blindValue(1 , $email); 
      $result->execute;
      // now you define the variable called $email like this 
      $email = isset($_POST['email']);
}

?>
<!DOCTYPE html>
<html lang="en">
<head>
      <meta charset="UTF-8">
      <meta http-equiv="X-UA-Compatible" content="IE=edge">
      <meta name="viewport" content="width=device-width, initial-scale=1.0">
      <title>php </title>
</head>
<body>
      <form action="" method="post">
            <input type="email" name="email" placeholder="Enter Your Email" id=""><br>
            <input type="submit" value="send" name="sub">
      </form>
</body>
</html>
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