gistfile1.php

<?php
// autofit_text_to_image() WordPress example
// http://cgd.io/2014/auto-fit-text-to-an-image-with-php-and-wordpress

$canvas_image_filename = 'quote_canvas.png';
$dest_filename = trailingslashit($upload_dir['path']) . "featured_image_{$post->ID}.jpg";
$text = 'I believe in pink. I believe that laughing is the best calorie burner. I believe in kissing, kissing a lot.';
$font_file = 'Cedarville-Cursive.ttf';

$result = autofit_text_to_image($canvas_image_filename, $dest_filename, $text, 60, 500, 400, 40, 400, $font_file);

if ($result) {
	$upload_dir = wp_upload_dir();
	$filetype = wp_check_filetype( basename( $dest_filename ), null );
		
	$attachment = array(
		'guid'           => $upload_dir['url'] . '/' . basename( $dest_filename ), 
		'post_mime_type' => $filetype['type'],
		'post_title'     => preg_replace( '/\.[^.]+$/', '', basename( $dest_filename ) ),
		'post_content'   => '',
		'post_status'    => 'inherit'
	);

	$new_attachment = wp_insert_attachment( $attachment, $dest_filename, $post->ID );
	$attach_data = wp_generate_attachment_metadata( $new_attachment, $dest_filename );
	wp_update_attachment_metadata( $new_attachment,  $attach_data );
		
	update_post_meta($post->ID, '_thumbnail_id', $new_attachment);
}