The PHP output buffering will save all the server outputs ( html and php prints) to a string variable.
So to start buffering, use ob_start(); this will keep saved any output.
Then you use $variable = ob_get_clean(); to stop buffering, and copy the buffer content to the variable.
Here are few samples of the use of ob_start() and ob_get_clean()
ob_start(); //Turn on output buffering ?>
//copy current buffer contents into $message variable and delete current output buffer
Hello world, link
Content
$var = ob_get_clean(); ?>