HTML dropdown and text form, php if statement?

essentially I have a database with a list of people & their ages. I would like to be able to read these, and I have set up a dropdown with either Name, and age as the attributes, then I have a text box where the user inputs either the name or age based on the choice in the dropdown.

<form target="_blank" action="results.php" method="post">
  <select name="attr">
    <option value="name">Name</option>
    <option value="age"> Age </option>
  </select>
<input type="text" id="name" name="name">
<input type="submit" name="submit" value="View Results">
</form>

Then in my results.php file I have if statements if(array_key_exists('name', $_POST)) or with age and then I am able to retrieve a table correctly. However, the above code is only working for name and not age, due to the 3rd last line. I basically want to set up an if statement here, if the selection in the dropdown is name or age, the text field to be name or age and retrieve it (which works correctly in my results.php). I have tried a switch statement and if/else to no avail. Any help would be appreciated!