php echo json command [duplicate] - php

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What does the PHP error message "Notice: Use of undefined constant" mean?
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"Notice: Undefined variable", "Notice: Undefined index", "Warning: Undefined array key", and "Notice: Undefined offset" using PHP
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when i try to Sign up echo json commands for existing email i get this error
Use of undefined constant Email_Error - assumed 'Email_Error' (this will throw an Error in a future version of PHP) in C:\xamppnew\htdocs\android\signup.php on line 16
{"SIGNUP":"Email_Error"}
and when there is no error i get this error
Notice: Undefined variable: arr in C:\xamppnew\htdocs\android\signup.php on line 23
null
php code
<?php
include "conn.php";
$Fname = $_POST['Fname'];
$Lname = $_POST['Lname'];
$Email = $_POST['Email'];
$Password = $_POST['Password'];
$sql_verify = "SELECT * FROM users WHERE Email = :EMAIL";
$stmt = $PDO->prepare($sql_verify);
$stmt->bindParam(':EMAIL', $Email);
$stmt->execute();
if ($stmt->rowCount() > 0) {
$arr = array( 'SIGNUP' => Email_Error);
}else{
echo "Signup compelete";
}
echo json_encode($arr);
?>
note im following php manual for json