Probá así:
Código PHP:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Documento sin título</title>
<script type="text/javascript">
var ns=[];
function ver(e){
var evt=e || event,str;
var t=evt.wich || evt.keyCode;
var o=evt.target || evt.srcElement;
if(document.selection){
str = document.selection.createRange().text;
}else{
var start = o.selectionStart;
var end = o.selectionEnd;
str = o.value.substring(start, end);
}
ns.sel=str;
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<textarea name="textarea" id="textarea" cols="45" rows="5" onmouseup="ver(event)">este es un ejemplo</textarea><input type="button" onClick="board.value=ns.sel || ''" value="copiar y pegar"><textarea name="board" cols="45" rows="5" id="board"></textarea>
</form>
</body>
</html>