<?php

/* Array with the full times list */
$startTimes = array();

/* Array with the already used times */
$arr = array();

/* This is the select HTML code */
$select = '<select name="mySelectName">';

/* Now, we build the options list */

foreach ($startTimes as $time)
{
    $select .= '<option value="' . htmlspecialchars($time) . '"';

    /* Disable the option if the time is used */
    if (in_array($time, $arr))
    {
        $select .= ' disabled="disabled"';
    }

    $select .= '>' . htmlspecialchars($time) . '</option>';
}

$select .= '</select>';
echo $select;