登录.php，检索和匹配密码不起作用

我有一个Login.php页面，可以检索并匹配注册用户的email address和password，如下所示：

<?php # login.php
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
// Initialize an error array.
$errors = array();
//Flag variables
$e = $p = FALSE;
// Trim all the incoming data:
$trimmed = array_map('trim', $_POST);
//Validate the email
if (!empty($trimmed['email'])) {
    $e = $trimmed['email'];
} else {
    $errors[] = 'You forgot to enter your email address!';
}
// Validate the password:
if (!empty($trimmed['pass'])) {
    $p = $trimmed['pass'];  
} else {
    $errors[]= 'You forgot to enter your password!';
}
if ($e && $p) { //OK
$q = "SELECT * FROM users WHERE email = ?";     
//Prepare the statement
    $stmt = $mysqli->prepare($q);
    //Bind the parameters
    $stmt->bind_param('s', $email) ;
    //Assign values to variables
    $email = $e;  
    //Execute the statement
    $stmt->execute();
    //Store the statement
    $stmt->store_result();
    $row_cnt = $stmt->num_rows;
        echo $row_cnt;
}else{// If everything wasn't OK.
    foreach ($errors as $msg) { // Print each error.
        echo "- $msg<br />'n";
        }
            echo 'Please try again!';
}//end of IF($e && $p)
}
?>
<form action="login.php" method="post" id="loginForm">
<fieldset>
    <p><b>Email</b><input type="email" name="email" ></p>
    <p><b>Password</b><input type="password" name="pass"> </p>
</fieldset>
<p><input type="submit"  value="Login" id="submit"></p>

一切都很顺利。但是，如果我将pass检查添加到select query中的 WHERE 中，如下所示：

$q = "SELECT * FROM users WHERE email = ? AND pass =?";
//Prepare the statement
    $stmt = $mysqli->prepare($q);
    //Bind the parameters
    $stmt->bind_param('ss', $email, $pass) ;
    //Assign values to variables
    $email = $e;
    $pass = SHA1('$p');

$row_cnt始终返回 0，即使我提交的密码与数据库中users表中的密码匹配或不匹配。

请问我做错了什么？我是否为 $pass 变量分配了正确的值，还是什么？

注意：我在注册过程中申请了$pass = SHA1('$p');。