<?PHP
    $content = "My name is Luka. I live on the second floor. I live upstairs from you. Yes I think you've seen me before. ";
    $dot = ".";

    $position = stripos ($content,$dot); //find first dot position

    if($position) { //if there's a dot in our soruce text do
        $offset = $position + 1; //prepare offset
        $position2 = stripos ($content,$dot,$offset); //find second dot using offset
        $first_two = substr($content,$position2); //put two first sentences under $first_two

        echo $first_two . '.'; //add a dot
    }

    else {  //if there are no dots
        //do nothing
    }
?>