Fatal error: Object of class stdClass could not be converted to string

Help me out, please. I (think I) fixed my plug-in’s bug but I don’t quite understand why. I was getting:

“fatal error: Object of class stdClass could not be converted to string” from this snippet:

if (preg_match('%POSTID%', $str_GaFormat))  {
   $str_GaFormat = str_ireplace('%POSTID%', $obj_PostID, $str_GaFormat);
}

Obviously (now), the $obj_PostID wasn’t a integer as it needed to be. So I did this:

if (preg_match('%POSTID%', $str_GaFormat)) {
  $row_Post = get_post($obj_PostID); // get the post
  $int_PostID = $row_Post->ID;
  $str_GaFormat = str_ireplace('%POSTID%', (string)$int_PostID, $str_GaFormat);
}

What’s not clear to me is why I was getting an error if the conditional wasn’t being met in the first place. In other words, in my test case(s) (preg_match('%POSTID%', $str_GaFormat)) was always false.

When a plug-in runs is there some sort of pre-processing? I know what you’re thinking, “What a noob…”. And you’re right! ?? My programming background goes back to the last 70s but PHP within WP is new to me. I’m learning, and often the hard way. I’d greatly appreciate anyone who might spend a moment educating me a bit on this issue.

Also, is there an easy (?) way to recognize what this problem is/was? Or is this just something I needed to learn?

Thanks in advance
Mark