Prueba con esto:
Código PHP:
<form method='post'>
<input type='checkbox' name='gender' value='0' <?php if(isset($_POST['gender']) && $_POST['gender'] == 0){ echo "checked = 'checked'"; } ?> > Sr.
<input type="checkbox" name="gender" value="1" <?php
if (isset($_POST['gender']) && $_POST['gender'] == 1) {
echo " checked='checked'";
}
?>>
</form>
Saludos