PHP - Retrieve Only That User Datas

CREATE TABLE IF NOT EXISTS `contacts` (
`contact_id` int(10) NOT NULL AUTO_INCREMENT,
`firstname` varchar(255) NOT NULL,
`lastname` varchar(255) NOT NULL,
`company` varchar(255) NOT NULL,
`cell_no` text NOT NULL,
PRIMARY KEY (`contact_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=latin1 AUTO_INCREMENT=8 ;

CREATE TABLE IF NOT EXISTS `users` (
`user_id` int(11) NOT NULL AUTO_INCREMENT,
`users_name` varchar(30) NOT NULL,
`uname` varchar(30) NOT NULL,
`u_pass` varchar(60) NOT NULL,
`utype` varchar(30) NOT NULL,
`timezone` varchar(30) NOT NULL,
`uapi_user` varchar(30) NOT NULL,
`uapi_pass` varchar(60) NOT NULL,
PRIMARY KEY (`user_id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 AUTO_INCREMENT=20 ;

<?php
if (isset($_POST['submit'])){
    //form has been submitted1
    $firstname  = trim($_POST['firstname']);
    $lastname   = trim($_POST['lastname']);
    $company    = trim($_POST['company']);
    $cellno = trim($_POST['cell_no']);

    if($firstname == ''){
        echo '<div class="alert alert-danger">First Name is not Valid!</div>';
        exit;
    }elseif($lastname == ''){   
        echo '<div class="alert alert-danger">Last Name is not Valid!</div>';
        exit;
    }elseif($company == ''){    
        echo '<div class="alert alert-danger">Company is not Valid!</div>';
        exit;
    }elseif($cellno == ''){ 
        echo '<div class="alert alert-danger">Cellphone Number is not Valid!</div>';
        exit;

    }else{

        $query = "Select cell_no from contacts where cell_no = '".$cellno."' ";
        $result = mysql_query($query);
        if (!mysql_num_rows($result)) {

        $sql = "INSERT INTO contacts(firstname, lastname, company, cell_no)
                            values('{$firstname}','{$lastname}', '{$company}', '{$cellno}')";
        $result = mysql_query($sql);
        confirm_query($result);

        //echo '<div class="alert alert-success">Successfully added.</div>';
        //exit;
        ?>
                    <script type="text/javascript">
            window.location = "contact_list.php";
        </script>
    <?php
    }
    else{

        echo '<div class="alert alert-danger">Username. already exist!.</div>';
        echo '<p><a href="new_contact.php" class="btn btn-success"> Back </a></p>';
        exit;

    }}

}else{
    $firstname  = "";
    $lastname   = "";
    $company    = "";
    $cellno = "";
}

?>

Am a newbie in php. Since I can't insert values to the database with respect to a user Id or with any other token using WHERE clause.

I.e "INSERT INTO receipts(date) VALUES(example) where id="**....."

If I need to fetch several values of column for a particular user, how do I go about it? Thank you!!!

elseif 
	($current_password !== $current_stored_password) {
	include 'includes/overall/header.php';
	echo $current_password . ' AND ' . $_POST['current_password'] . ' Password and password again do not match';
	include 'includes/overall/header.php';

}

<?php
session_start();
error_reporting(0);
//ini_set('display_errors', '1');

require( 'database.php' );

$username = $_SESSION['loggedinuser'];
$current_stored_password = $_SESSION['password'];
$current_password = $_POST['current_password'];
$password = mysqli_real_escape_string($con, md5( $_POST['password']));
$password_again = mysqli_real_escape_string($con, md5( $_POST['password_again']));


// Run checks

if	(isset($_POST['current_password'], $_POST['password'], $_POST['password_again'])) {
	
if( strlen( $_POST['current_password'] ) < 8 )
    {
        include('includes/overall/header.php');
        echo "Password Must Be 8 or More Characters.";
        include('includes/overall/footer.php');
}
elseif( strlen( $_POST['password'] ) < 8 )
    {
        include('includes/overall/header.php');
        echo "Password Must Be 8 or More Characters.";
        include('includes/overall/footer.php');
    }
elseif 
		( strlen( $_POST['password_again'] ) < 8 )
    {
        include('includes/overall/header.php');
        echo "Password Must Be 8 or More Characters.";
        include('includes/overall/footer.php');
    }
elseif 
	($password !== $password_again) {
	include 'includes/overall/header.php';
	echo ' Password and password again do not match';
	include 'includes/overall/header.php';
}
elseif 
	($current_password !== $current_stored_password) {
	include 'includes/overall/header.php';
	echo $current_password . ' AND ' . $_POST['current_password'] . ' Password and password again do not match';
	include 'includes/overall/header.php';

} else {

// Define a query to run 
$query = "UPDATE `user` SET `password` = '$password' WHERE `username` = '$username'"; 

// Query the database 
$result = mysqli_query($con,$query); 

// Check if the query failed 
if( !$result ) 
{ 
   die('There was a problem executing the query ('.$query.'):<br>('.mysqli_errno($con).') '.mysqli_error($con)); 
} 

else {

include 'includes/overall/header.php';
	echo 'Password has been changed';
include 'includes/overall/footer.php';
}    
}
}

// Close the connection 
mysqli_close($con); 
?> 

	echo $current_password . ' AND ' . $_POST['current_password'] . ' Password and password again do not match';