<?php
$YAZI = "Merhaba, bu, yazıda, toplam, {virgul_sayisi}, virgül, var.";
$VIRGUL_SAYISI = substr_count( $YAZI, "," );
$YAZI_SON = str_replace( "{virgul_sayisi}", $VIRGUL_SAYISI, $YAZI );

echo $YAZI_SON;
?>