need help inserting a default text value into mysql

Question

end web developer, i was given a CMS done from another team and i have to link with my front-end. I have made some modifications, but due to my lack of php knowledge i have some issue here.

My users are able to fill up a form, where 1 text field is asking for their photo link. I want to check for if the value entered is not equal to what i want, then i will query insert a default avatar photo link to mysql to process.

code that i tried on php

// check if the variable $photo is empty, if it is, insert the default image link
if($photo = ""){
    $photo="images/avatarDefault.png";
}

doesn't seem to work

<?php
if($_SERVER["REQUEST_METHOD"] === "POST")
{
    //Used to establish connection with the database
    include 'dbAuthen.php';
    if (mysqli_connect_errno())
    {
        echo "Failed to connect to MySQL: " . mysqli_connect_error();
    }
    else
    {

        //Used to Validate User input
        $valid = true;

        //Getting Data from the POST
        $username = sanitizeInput($_POST['username']);
        $displayname = sanitizeInput($_POST['displayname']);
        $password = sanitizeInput($_POST['password']);

        //hash the password using Bcrypt - this is to prevent 
        //incompatibility from using PASSWORD_DEFAULT when the default PHP hashing algorithm is changed from bcrypt  
        $hashed_password = password_hash($password, PASSWORD_BCRYPT);

        //Determining Type of the User
        //if B - User is student
        //if A - User is adin
        if($_POST['type'] == 'true')
            $type = 'B';
        else
            $type = 'A';

        $email = sanitizeInput($_POST['email']);
        $tutorGroup = sanitizeInput($_POST['tutorGroup']);
        $courseID = sanitizeInput($_POST['courseID']);
        $description = sanitizeInput($_POST['desc']);
        $courseYear = date("Y");
        $website = sanitizeInput($_POST['website']);
        $skillSets = sanitizeInput($_POST['skillSets']);
        $specialisation = sanitizeInput($_POST['specialisation']);
        $photo = sanitizeInput($_POST['photo']);

        // this is what i tried, checking if the value entered is empty, but doesn't work
        if($photo = ""){
            $photo="images/avatarDefault.png";
        }

        $resume = sanitizeInput($_POST['resume']);

        //Validation for Username
        $sql = "SELECT * FROM Users WHERE UserID= '$username'";
        if (mysqli_num_rows(mysqli_query($con,$sql)) > 0){
            echo 'User already exists! Please Change the Username!<br>';
            $valid = false;
        }

        if($valid){
            //Incomplete SQL Query
            $sql = "INSERT INTO Users
             VALUES ('$username','$displayname','$hashed_password','$type','$email', '$tutorGroup', ";

            //Conditionally Concatenate Values
            if(empty($courseID))
            {
                $sql = $sql . "NULL";
            }
            else
            {
                $sql = $sql . " '$courseID' ";
            }

            //Completed SQL Query
            $sql = $sql . ", '$description', '$skillSets', '$specialisation', '$website', '$courseYear', '$photo',  '$resume', DEFAULT)";

            //retval from the SQL Query
            if (!mysqli_query($con,$sql))
            {
                echo '*Error*: '. mysqli_error($con);
            }
            else
            {
                echo "*Success*: User Added!";
            }
        }

        //if student create folder for them
        if ($type == 'B')
        {
            //Store current reporting error
            $oldErrorReporting = error_reporting();

            //Remove E_WARNING from current error reporting level to prevent users from seeing code
            error_reporting($oldErrorReporting ^ E_WARNING);

            //Set current reporting error();
            error_reporting($oldErrorReporting);
        }

        mysqli_close($con);
    }
}
function sanitizeInput($data)
{
    $data = trim($data);
    $data = stripslashes($data);
    $data = htmlspecialchars($data);
    return $data;
}
?>

i've tried finding a way on mysql to insert default values but it seem impossible, so i have no choice but to query insert through php.

I have the logic but i'm not sure how to implement on the php with my lack of knowledge, i was thinking of checking either 1) if the photo link does not have the word .png/.jpg, $photo != ".png" 2) if the photo link length is too low $.photo.length < 10

can someone help me look into the code and tell me what i'm doing wrong? Thanks!

Answer 1

First thing that I notice is to use double =

if($photo == ""){
 //...
}

Answer 2

A very simple way with default values could be:

$photo = isset($photo) ? $photo : 'images/avatarDefault.png' ;

How it works is that it first it asks if the photo is set, if it is, use all ready inserted value, otherwise insert your default value,

Another (very alike) method to use:

$photo = !empty($photo) ? $photo : 'images/avatarDefault.png' ;

UPDATE

To check if it contains a certain "extension" would be a simple rewrite

$photo = preg_match('#\b(.jpg|.png)\b#', $photo ) ? $photo : "images/avatarDefault.png" ;

This way it checks wether the text / image link in $photo contains the .png file type, if it doesn't it inserts your default image