Wordpress how to check if a post is the first post

I am using the get_previous_post() to check if the first custom post type so I can add a condition. It works if it's not equal to 0 (the first post), to make it work I need to parse the output.

The following didn't work, I get the error Argument ($str) must be of type string

$checkFirst = get_previous_post();
<?php !(wp_parse_str(strlen($checkFirst)) == 0) ? 'something' : '';?>

I have also tried :

wp_parse_str(strlen($checkFirst), $output)
<?php !($output[strlen($checkFirst)] == 0) ? 'something' : '';?>

Why do you think you need to parse it?

According to the get_previous_post^Docs, it returns:

• Null if global $post is not set.
• Empty string if no corresponding post exists.
• Post object if the current post is not the first one.

So you could check its value like this:

$checkFirst = get_previous_post();

echo $checkFirst  
               ? 'There is a previous post' 
               : 'No, this is the first post';

You don't need to use strlen or wp_parse_str.