phpThumb unable to add watermark

Question

I am trying to add a watermark on an image using phpThumb in PHP8.2 and getting following error:

$gdimg_hexcolorallocate is not a GD resource in ImageHexColorAllocate()

Following is the code which I have tried:

include_once 'phpThumb/phpthumb.class.php';
    
    $phpThumb                = new phpThumb(); 
    
    $phpThumb->setSourceFilename( dirname( __FILE__ ) . '/phpThumb/images/img-captcha.png' ); 
     
    $font_file = dirname( __FILE__ ) . '/phpThumb/fonts/opensans-extrabold-webfont.ttf';
    
    $phpThumb->setParameter( 'fltr',array('wmt|test|13.5|C|#000000|'.$font_file.'|100|5|0|#000000||1.0'));
    
    $phpThumb->setParameter( 'w', '200' );
    
    $phpThumb->setParameter( 'h', '100' );
    
    $phpThumb->setParameter( 'config_output_format', 'png' );
    
    $image_name      = 'captcha_' . time() . '_' . $thumbnail_width . '.' . $phpThumb->config_output_format;
    
    $output_filename = $base_path . '/cache/captcha/' . $image_name;
    
    $phpThumb->GenerateThumbnail();
    
    $phpThumb->RenderToFile( $output_filename );