function janela(mypage, myname, w, h) {
   var winl = (screen.width - w) / 2;
   var wint = (screen.height - h) / 2;
   winprops = 'height='+h+',width='+w+',top='+wint+',left='+winl+',scrollbars=YES,noresizable,status=yes, location=no, toolbar=no, menubar=no';
   win = window.open(mypage, myname, winprops)
}

function popUp() {
	janela("alerta.php", "POPUP",  620, 550);
}