PHP Image hashing

I am making a social network and I want to give the users profile picture a unique name when they upload it to the folder directory and/or the database . I looked on here and saw some good answers but I don't know how to implement them with my code . I tried this code $strtotime = strtotime("now"); $filename = tempnam ($strtotime.'_'.$_FILES['file']['name']); but that changed nothing . The images still aren't hashed . Can someone help me ?

upload.php:

$target_dir = "images/uploads/";
$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
$uploadOk = 1;
$imageFileType = pathinfo($target_file,PATHINFO_EXTENSION);

$strtotime = strtotime("now");
$filename = tempnam ($strtotime.'_'.$_FILES['file']['name']);

// Check if image file is a actual image or fake image

if(isset($_POST["change"])) {

move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file);
$sql = "UPDATE users SET userPic = '".$_FILES['fileToUpload']['name']."' WHERE username = '" . $username . "'";

$check = $conn->query($sql);

if($check !== false) {
echo "<a href = profile.php> Profile pciture has been changed </a>" . 
 $check["mime"] . ".";
$uploadOk = 1;

} else {
echo "File is not an image.";
$uploadOk = 0;
}
} else {

echo"did not change";
}

EDIT :

<form id="form2" action="upload.php" method="post" enctype="multipart/form-data">
<p id="p1">Change profile picture:</p> <br />
<input type="file" name="fileToUpload" id="fileToUpload"><br />
<br><input id="sub1" type="submit" value="Change profile picture" name="change"><br />
</form>

<!-- Trigger the Modal -->
<img id="myImg" src="default.png" width="200" height="150">

<!-- The Modal -->
<div id="myModal" class="modal">

 <!-- The Close Button -->
<span class="close" onclick="document.getElementById('myModal').style.display='none'">&times;</span>

<!-- Modal Content (The Image) -->
<img class="modal-content" id="img01">

<!-- Modal Caption (Image Text) -->
<div id="caption"></div>
</div>
<script>
// Get the modal
var modal = document.getElementById('myModal');

// Get the image and insert it inside the modal - use its "alt" text as a 
caption
var img = document.getElementById('myImg');
var modalImg = document.getElementById("img01");
var captionText = document.getElementById("caption");
img.onclick = function(){
modal.style.display = "block";
modalImg.src = this.src;
captionText.innerHTML = this.alt;
}

// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];

// When the user clicks on <span> (x), close the modal
span.onclick = function() { 
modal.style.display = "none";
}
</script>