<script type="text/javascript">
function ModifyPlaceHolder1 () {
var input = document.getElementById ("MyQuery");
input.placeholder = "Search books e.g. Harry Potter";
}
function ModifyPlaceHolder2 () {
var input = document.getElementById ("MyQuery");
input.placeholder = "Search journals e.g. New Scientist";
}
</script>


<input type="text" id="MyQuery" placeholder="Search resources" name="q" />
<input type="radio" value="" id="All" name="s.cmd" checked="checked" />
<label for="All">All</label>
<input type="radio" onclick="ModifyPlaceHolder1 ()" value="" id="Books" name="s.cmd" checked="checked" />
<label for="Books">Books</label>
<input type="radio" onclick="ModifyPlaceHolder2 ()" value="" id="Journals" name="s.cmd" checked="checked" />
<label for="Journals">Journals</label>

<input type="text" id="MyQuery" placeholder="Search resources" name="q" />
<input type="radio" value="" id="All" name="s.cmd" checked="checked" />
<label for="All">All</label>
<input type="radio" value="" id="Books" name="s.cmd" checked="checked" />
<label for="Books">Books</label>
<input type="radio" value="" id="Journals" name="s.cmd" checked="checked" />
<label for="Journals">Journals</label>

var books = document.getElementById("Books");
var journals = document.getElementById("Journals");
var input = document.getElementById("MyQuery");

books.onclick = function() {
    input.placeholder = "Search books e.g. Harry Potter";
}

journals.onclick = function() {
    input.placeholder = "Search journals e.g. New Scientist";
}

<script>
function ModifyPlaceHolder(element) {
  var data = {
    All: 'Search resources',
    Books: 'Search books e.g. Harry Potter',
    Journals: 'Search journals e.g. New Scientist'
  };
  var input = element.form.MyQuery;
  input.placeholder = data[element.id];
}

</script>

<form>
  <input type="text" id="MyQuery" placeholder="Search resources" name="q">
  <label for="All"><input type="radio" onclick="ModifyPlaceHolder(this)" id="All" name="s.cmd" checked> All</label>
  <label for="Books"><input type="radio" onclick="ModifyPlaceHolder(this)" id="Books" name="s.cmd"> Books</label>
  <label for="Journals"><input type="radio" onclick="ModifyPlaceHolder(this)" id="Journals" name="s.cmd"> Journals</label>
</form>

`<script>
  function ModifyPlaceHolder(element) {
  var data = {
  All: 'Search resources',
  Books: 'Search books e.g. Harry Potter',
 Journals: 'Search journals e.g. New Scientist'
  };
var input = document.getElementById("MyQuery");
input.placeholder = data[element.id];
}
</script>`