You can use curl_multi_getcontent() on a curl handle that was executed with curl_exec() (and not added to a multi handle).
However, this is not very useful because curl_multi_getcontent() will return the same as curl_exec() then.
<?php
$ch = curl_init('http://www.example.com/');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
$a = curl_exec($ch);
$b = curl_multi_getcontent($ch);
var_dump($a === $b);
curl_close($ch);
?>
will return:
bool(true)