<?php
ob_start();
echo 'a';
print 'b';
// some statement that removes all printed/echoed items
ob_end_clean();
echo 'c';
// the final output is equal to 'c', not 'abc'
?>
The output buffering functions are also useful in hackery to coerce functions that only print to return strings, ie.
<?php
ob_start();
var_dump($myVar);
$data = ob_get_clean();
// do whatever with $data
?>