//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>