How to include a custom function file

I would like to use the following code on one of my pages. I hard-coded it but the file is not displaying. How do I achieve this?

// Set up the airport variables
		$toairport = $result['destination_addresses'][0];
		$fromairport= $result['origin_addresses'][0];

			/**
			 * Check if the location is an airport.
			 *
			 * @param string $location
			 * @return bool
			 */
			function isAirport($location)
			{
			    $airportList = ['Heathrow', 'Gatwick', 'London Luton', 'London City Airport'];
			    foreach ($airportList as $airport) {
			        if (strpos($location, $airport) !== false) {
			            return true;
			        }
			    }

			    return false;
			}
if (isAirport($toairport) || isAirport($fromairport)) { ?>
    <div class="airport-transfer">
    	<p>We see that your journey involves a London airport transfer.</p>
    	<p>On the meter, the cost can range from <b>
    		<?php
    		if ($Distance <= intval(1454)) {
    			echo $DayMinDistanceCost . " </b>to<b> " .$FestiveMinDistanceCost;
    		} else {
    			echo $DayCost . " </b>to<b> " .$FestiveCost;
    		} ?>
    	</b>or higher, depending on traffic.</p>
    	<p>We may be able to offer you a competitive flat rate. Please contact us on <b>01252-83-40-50</b> and ask for a London Airport Transfer rate.</p>
    </div>
    <div class="another-quote">Get another <a href="<?php echo $_SERVER['PHP_SELF'];?>">quote</a></div>
<?php }