HTML checked="checked" and selected="selected"

We use checked="checked" in radio button (circle) or checkbox (square) to indicate an item is checked.
We use selected="selected" in a drop-down menu to indicate an item is selected.
Combination with PHP, we can edit  user file etc using MySQL database:
Example:
<input type="radio" name="email_admin" value="1"  <?php if($row['email_admin'] == 1){ echo('selected="selected"'); } ?>/>Enable

Example of   checked="checked" in radio button:
<form>
<input type="radio" name="car" value="Nissan" checked="checked" /> Nissan<br />
<input type="radio" name="car" value="Toyota" /> Toyota
</form>
 Example of   checked="checked" in checkbox:
<form>
<input type="checkbox" name="car" value="Nissan" checked="checked" /> Nissan<br />
<input type="checkbox" name="car" value="Toyota" /> Toyota
</form>
Example of  selected="selected" in a drop-down list:
<select name="currency">
<option value="CAD" >CAD - Canadian Dollars</option>
<option value="AUD" selected="selected">AUD - Australian Dollars</option>
<option value="BRL">BRL - Brazilian Reais</option>
<option value="GBP">GBP - British Pounds</option>
<option value="CZK">CZK - Czech Koruny</option>
<option value="DKK">DKK - Danish Kroner</option>
</select>