Probá así:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>ejemplo</title>
<script>
function sel(inicio,fin,idCampo){
input=document.getElementById(idCampo);
if(typeof document.selection != 'undefined' && document.selection){
tex=input.value;
input.value='';
input.focus();
var str = document.selection.createRange();
input.value=tex;
str.move('character', inicio);
str.moveEnd("character", fin-inicio);
str.select();
}
else if(typeof input.selectionStart != 'undefined'){
input.setSelectionRange(inicio,fin);
input.focus();
}
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<textarea name="area" cols="60" rows="10" id="area">Qué lindo perro.</textarea>
<input type="button" name="Submit" value="-3" onclick="sel(document.getElementById('area').value.length-4,document.getElementById('area').value.length-4,'area')" /><input type="button" name="Submit" value="-5" onclick="sel(document.getElementById('area').value.length-6,document.getElementById('area').value.length-6,'area')" />
</form>
</body>
</html>