Question

How can I process the checkboxes only if they’re checked and grab the value of the checked ones only.

php

if (is_array($_POST['add'])) {
  foreach ($_POST['add'] as $key => $value) {
    $_POST['add'][$key] = mysql_real_escape_string(stripslashes($value));
  }
}

html

<input type="checkbox" id="wmeet_ce" 
        value="ce" 
        name="add[wmeet]" 
        title="Wanting To Meet" 
        class="checkbox {validate:{required:true,minlength:1}}"/>

<input type="checkbox" id="wmeet_sf"
        value="sf" 
        name="add[wmeet]" 
        class="checkbox"/>

<input type="checkbox" id="wmeet_sm" 
        value="sm" 
        name="add[wmeet]" 
        class="checkbox" />

Answer 1

Only checked checkboxes are ever presented to PHP, so your PHP code is correct.

However, your HTML isn’t correct, as all your checkboxes have the same name. This means PHP will only ever see one of them.

To get an array of checkboxes you either need to give your checkboxes unique names like this

<input type="checkbox" id="wmeet_ce" 
        value="ce" 
        name="add[ce]" 
        title="Wanting To Meet" 
        class="checkbox {validate:{required:true,minlength:1}}"/>

<input type="checkbox" id="wmeet_sf"
        value="sf" 
        name="add[sf]" 
        class="checkbox"/>

<input type="checkbox" id="wmeet_sm" 
        value="sm" 
        name="add[sm]" 
        class="checkbox" />

Or use the empty box technique like this.

<input type="checkbox" id="wmeet_ce" 
        value="ce" 
        name="add[]" 
        title="Wanting To Meet" 
        class="checkbox {validate:{required:true,minlength:1}}"/>

<input type="checkbox" id="wmeet_sf"
        value="sf" 
        name="add[]" 
        class="checkbox"/>

<input type="checkbox" id="wmeet_sm" 
        value="sm" 
        name="add[]" 
        class="checkbox" />