需要帮助将默认文本值插入 MySQL

>end Web开发人员，我得到了另一个团队完成的CMS，我必须与我的前端链接。我做了一些修改，但由于我缺乏 php 知识，我在这里遇到了一些问题。

我的用户可以填写一个表单，其中 1 个文本字段要求提供他们的照片链接。我想检查输入的值是否等于我想要的值，然后我将查询插入默认头像照片链接到mysql进行处理。

我在 PHP 上尝试过的代码

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

似乎不起作用

<?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;
}
?>

我尝试在MySQL上找到一种插入默认值的方法，但这似乎是不可能的，所以我别无选择，只能通过PHP查询插入。

我有逻辑，但我不确定如何在 php 上实现我缺乏知识，我正在考虑检查1） 如果照片链接没有.png/.jpg字样，$photo != ".png"2）如果照片链接长度太低$.photo.length < 10

有人可以帮助我查看代码并告诉我我做错了什么吗？谢谢！