index.php

<?php
session_start();

include('includes/functions.php');

if( isset( $_POST['login'] ) ) {
    // create variables
    // wrap data with validate function
    $formEmail = validateFormData( $_POST['email'] );
    $formPass = validateFormData( $_POST['password'] );
    
    // connect to database
    include('includes/connection.php');
    
    // create query
    $query = "SELECT name, password FROM users WHERE email='$formEmail'";
    
    // store the result
    $result = mysqli_query( $conn, $query );
    
    // verify if result is returned
    if( mysqli_num_rows($result) > 0 ) {
        
        //store basic user data in variables
        while( $row = mysqli_fetch_assoc($result) ) {
            $name = $row['name'];
            $hashedPass = $row['password'];
        }
        
        // verify hashed password with submitted password
        if( password_verify( $formPass, $hashedPass ) ) {
            
            // correct login details
            // store data in SESSION variables
            $_SESSION['loggedInUser'] = $name;
            
            // redirect user to clients page
            header( "Location: clients.php" );
        } else { //hashed password didn't verify
            
            // error message
            $loginError = "<div class='alert alert-danger'>Wrong username/password. Try again.</div>";
        }
        
    } else { // there are no result in database
        
        // error message
        $loginError = "<div class='alert alert-danger'>No such user in database. Try again.<a class='close' data-dismiss='alert'>&times;</a></div>";
    }
    
}

// close mysql connection
mysqli_connect($conn);

include('includes/header.php');
?>

<h1>Client Address Book</h1>
<p class="lead">Log in to your account.</p>

<?php echo $loginError; ?>

<form class="form-inline" action="<?php echo htmlspecialchars( $_SERVER['PHP_SELF'] ); ?>" method="post">
    <div class="form-group">
        <label for="login-email" class="sr-only">Email</label>
        <input type="text" class="form-control" id="login-email" placeholder="email" name="email" value="<?php echo $formEmail; ?>">
    </div>
    <div class="form-group">
        <label for="login-password" class="sr-only">Password</label>
        <input type="password" class="form-control" id="login-password" placeholder="password" name="password">
    </div>
    <button type="submit" class="btn btn-primary" name="login">Login</button>
</form>

<?php
include('includes/footer.php');
?>