$icerik = file_get_contents('LİNKİ BURAYA YAZIN');

preg_match_all('@<p>(.*?)</p>@i', $icerik, $eslesmeler);

$en_uzun_metin = '';

foreach ($eslesmeler[1] as $metin)
   if (strlen($metin) > strlen($en_uzun_metin))
      $en_uzun_metin = $metin;

echo $en_uzun_metin;