How to use a PHP-generated image inside the FFMPEG?

I have a PHP file that generates an image. So the result of the following code would be an image shown in the browser:

<?php
require_once('generateImg.php');`

Now I need to use that generated image instead of img.jpeg in the following command:

// This command successfully generates `output.mp4` file
exec('ffmpeg  -loop 1 -t 1 -i img.jpeg -i audio.mp3  -f flv -vf  scale=640:ih*640/iw -shortest output.mp4');

Here is what I’ve tried:

ob_start();
require_once('generateImg.php');
$img = ob_get_clean();

exec("ffmpeg  -loop 1 -t 1 -i $img -i audio.mp3  -f flv -vf  scale=640:ih*640/iw -shortest output.mp4");

But it doesn’t generate the output.mp4 file. Any idea how can I do that?

Here’s the content of generateImg.php file:

<?php

$image = imagecreate(500, 300);
$background_color = imagecolorallocate($image, 0, 153, 0);
$text_color = imagecolorallocate($image, 255, 255, 255);

imagestring($image, 5, 180, 100,  "GeeksforGeeks", $text_color);
imagestring($image, 3, 160, 120,  "A computer science portal", $text_color);

header("Content-Type: image/png");

imagepng($image);
imagedestroy($image);