<?php
  $texto= "Vamos a reemplazar la www.centroholistico.org palabra por la nueva";

  $texto  =  str_replace ( "\r" ,  "" ,  $texto ); 

  $texto  =  preg_replace ( "/(www.[^(\)|[:space:])]*)\)/" , "<a href=\"http://\\1\" target=\"_blank\">\\1</a>)" , $texto ); 

  $texto  =  preg_replace ( "/(www.[^([:space:]|\"|\<)]*)[[:space:]]/" , "<a href=\"http://\\1\" target=\"_blank\">\\1</a> " , $texto ); 

  $texto  =  preg_replace ( "/\<a href=\"http:\/\/(.*zonasiete.*)\"  target=\"_blank\"\>/" , "<a href=\"http://\\1\">" , $texto ); 

  echo $texto;

?>