Careful with while using functions that change headers of a page; that change will not be undone when ending output buffering.
If you for instance have a class that generates an image and sets the appropriate headers, they will still be in place after the end of ob.
For instance:
<?php
 ob_start();
 myClass::renderPng(); //header("Content-Type: image/png"); in here
 $pngString = ob_get_contents();
 ob_end_clean();
?>

will put the image bytes into $pngString, and set the content type to image/png. Though the image will not be sent to the client, the png header is still in place; if you do html output here, the browser will most likely display "image error, cannot be viewed", at least firefox does.
You need to set the correct image type (text/html) manually in this case.