Want to change password

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PHP

Change password but first match current password and the update the new password. But I am getting error updating password with this code. Please help me.

<?php
	//session_start();
	//connect to database
	include "../pages/connect.php";
	//$db = mysql_connect("localhost", "root", "rangers", "doctor");
	/*if (!$db) {
    die('Could not connect: ' . mysql_error());
}
echo 'Connected successfully';
mysql_close($db);*/


	if(isset($_POST['change_password_btn']))
	{
		$username = mysql_real_escape_string($_POST['user_id']);
		$password = mysql_real_escape_string($_POST['password']);
		$newpassword = mysql_real_escape_string($_POST['passwordnew']);
		
		if($row['password']==($password))
		{
		// update data in mysql database 
$sql="UPDATE registerdoctors SET password='$newpassword' WHERE doc_id='$username'" or die ("this stuffed up");
$result=mysql_query($sql) or die ("this stuffedup");
}

// if successfully updated. 
if($result){
//echo "Successful";
	?>
			  <script type="text/javascript">
              alert("password updated successfully.");
              window.location.href = '../pages/doctorwelcome.php?page=changepassword';
              </script>
              <?php
}

else {
//echo "error";
	?>
	 			<script type="text/javascript">
              alert("error updating...");
              window.location.href = '../pages/doctorwelcome.php?page=changepassword';
              </script>
              <?php

}




	
	}

	?>

What I have tried:

Change password but first match current password and the update the new password. But I am getting error updating password with this code. Please help me.