How to rename uploaded file before saving it into a directory?

You guess correctly. Read the manual page for move_uploaded_file. Set the second parameter to whereever your want to save the file.

If it doesn't work, there is something wrong with your $fileName. Please post your most recent code.

You can simply change the name of the file by changing the name of the file in the second parameter of move_uploaded_file.

Instead of

move_uploaded_file($_FILES["file"]["tmp_name"], "../img/imageDirectory/" . $_FILES["file"]["name"]);

Use

$temp = explode(".", $_FILES["file"]["name"]);
$newfilename = round(microtime(true)) . '.' . end($temp);
move_uploaded_file($_FILES["file"]["tmp_name"], "../img/imageDirectory/" . $newfilename);

Changed to reflect your question, will product a random number based on the current time and append the extension from the originally uploaded file.

/* create new name file */
$filename   = uniqid() . "-" . time(); // 5dab1961e93a7-1571494241
$extension  = pathinfo( $_FILES["file"]["name"], PATHINFO_EXTENSION ); // jpg
$basename   = $filename . "." . $extension; // 5dab1961e93a7_1571494241.jpg

$source       = $_FILES["file"]["tmp_name"];
$destination  = "../img/imageDirectory/{$basename}";

/* move the file */
move_uploaded_file( $source, $destination );

echo "Stored in: {$destination}";