$max_chars = 100;
$readmore = '[readmore]'; // Suffix ini hanya akan muncul jika posting > 100

$sql = mysql_query("SELECT id, judul, IF(CHAR_LENGTH(LEFT(posting, $max_chars+1))>$max_chars, CONCAT(LEFT(posting, $max_chars), '$readmore'), posting) AS posting FROM tabel_posting LIMIT 10");

while($row = mysql_fetch_array($sql)) {

$id = $row['id'];
$judul = $row['judul'];
$posting = $row['posting'];

if (substr($posting, -strlen($readmore)) == $readmore) { // Jika ada [readmore] di belakang, replace dengan link ..selengkapnya
$posting = str_replace($readmore, '<a href="/posting.php?id='.$id.'">..selengkapnya</a>', $posting);
}

// blabla

echo htmlspecialchars($posting);

}