Buscá por charAt (
http://www.w3schools.com/jsref/jsref_charAt.asp): 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=utf-8" />
<title>Documento sin título</title>
</head>
<body>
<form action="" method="get">
<input value="abcd" name="campo" type="text" />
<input type="button" value="0" onclick="alert(campo.value.charAt(value))" />
<input type="button" value="1" onclick="alert(campo.value.charAt(value))" />
<input type="button" value="2" onclick="alert(campo.value.charAt(value))" />
<input type="button" value="3" onclick="alert(campo.value.charAt(value))" />
</form>
</body>
</html>
La cantidad de caracteres corresponde a la propiedad length:
http://www.w3schools.com/jsref/jsref_length_string.asp