Use Font Awesome Icon in Placeholder

@Elli's answer can work in FontAwesome 5, but it requires using the correct font name and using the specific CSS for the version you want. For example when using FA5 Free, I could not get it to work if I included the all.css, but it worked fine if I included the solid.css:

<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.8.1/css/solid.css">

<input type="text" placeholder="&#xF002; Search" style="font-family: Arial, 'Font Awesome 5 Free'" />

For FA5 Pro the font name is 'Font Awesome 5 Pro'

You can't add an icon and text because you can't apply a different font to part of a placeholder, however, if you are satisfied with just an icon then it can work. The FontAwesome icons are just characters with a custom font (you can look at the FontAwesome Cheatsheet for the escaped Unicode character in the content rule. In the less source code it's found in variables.less The challenge would be to swap the fonts when the input is not empty. Combine it with jQuery like this.

<form role="form">
  <div class="form-group">
    <input type="text" class="form-control empty" id="iconified" placeholder="&#xF002;"/>
  </div>
</form>

With this CSS:

input.empty {
    font-family: FontAwesome;
    font-style: normal;
    font-weight: normal;
    text-decoration: inherit;
}

And this (simple) jQuery

$('#iconified').on('keyup', function() {
    var input = $(this);
    if(input.val().length === 0) {
        input.addClass('empty');
    } else {
        input.removeClass('empty');
    }
});

The transition between fonts will not be smooth, however.

If you're using FontAwesome 4.7 this should be enough:

<link href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet"/>

<input type="text" placeholder="&#xF002; Search" style="font-family:Arial, FontAwesome" />

A list of hex codes can be found in the Font Awesome cheatsheet. However, in the lastest FontAwesome 5.0 this method does not work (even if you use the CSS approach combined with the updated font-family).

I solved with this method:

In the CSS I used this code for the fontAwesome class:

.fontAwesome {
  font-family: 'Helvetica', FontAwesome, sans-serif;
}

In the HTML I have added the fontawesome class and the fontawesome icon code inside the placeholder:

<input type="text" class="fontAwesome" name="emailAddress" placeholder="&#xf0e0;  insert email address ..." value="">

You can see in CodePen.