default thumbnail for all posts in same category - wordpress

Question

My Wordpress site use first image as post thumbnail, code:
add_filter('the_content','replace_content');
function get_first_image() {
global $post, $posts;
$first_img = '';
ob_start();
ob_end_clean();
$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
$first_img = $matches[1][0];
if(empty($first_img)) {
$first_img = "/path/to/default.png";
}
return $first_img;
}
Some posts have no image in its content, so i want to use different default thumbnails for them: posts in category A use picture1, posts in cagory B use category2...
How can I do this?
thank you

Answer 1

You need to get the current post category first, then, make a if statement in your code.
Check this link : https://developer.wordpress.org/reference/functions/get_the_category/#comment-305

Answer 2

How about this, here we are using has_post_thumbnail to check for image attachments, if none exist we are setting up the image and image source. From there we check for category assignment, if there are no category matches we use a default thumbnail.
<?php
if ( ! has_post_thumbnail() ) {
$themefolder = get_bloginfo('template_directory');
echo '<img src="' . $themefolder . '/images/';
if ( is_category( 'Category A' ) ) {
echo 'no-image-a.png';
} elseif ( is_category( 'Category B' ) ) {
echo 'no-image-b.png';
} else {
echo 'no-image.png';
}
echo '" alt="' . get_the_title() . '">' . PHP_EOL;
}
?>

Answer 3

Finally, It's my code:
add_filter('the_content','replace_content');
function get_first_image() {
global $post, $posts;
$first_img = '';
ob_start();
ob_end_clean();
$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
$first_img = $matches[1][0];
if(empty($first_img)) {
$categories = get_the_category();
if ( ! empty( $categories ) ) {
foreach ( $categories as $category ) {
if( $category->name == 'Category A' ){ $first_img = "/path/to/default1.png";}
elseif ( $category->name == 'Category B' ){ $first_img = "/path/to/default2.png";}
else {$first_img = "/path/to/default3.png";}
}
}
}
return $first_img;
}