Here is a quick post on converting from KML files used in Google Earth/Maps to GPX Exchange Format (GPX) with PHP.

Code is quite explanatory, change $u with the location of the KML file, code will output GPX XML.

Alternatively, download the code here.

<?php
	//enter location of KML file here
	$u = "http://code.google.com/apis/kml/documentation/KML_Samples.kml";
	
	
	function utcdate() {
		return gmdate("Y-m-d\Th:i:s\Z");
	}

	
	$u_parts = pathinfo($u); //array of url parts
	$u_ext = strtoupper($u_parts['extension']);
	if ($u_ext== "KML") {

		$dom_kml = new DOMDocument();
		$dom_kml->load($u);

		$dom_gpx = new DOMDocument('1.0', 'UTF-8');
		$dom_gpx->formatOutput = true;
		
		//root node
		$gpx = $dom_gpx->createElement('gpx');
		$gpx = $dom_gpx->appendChild($gpx);
		
		$gpx_version = $dom_gpx->createAttribute('version');
		$gpx->appendChild($gpx_version);
		$gpx_version_text = $dom_gpx->createTextNode('1.0');
		$gpx_version->appendChild($gpx_version_text);
		
		$gpx_creator = $dom_gpx->createAttribute('creator');
		$gpx->appendChild($gpx_creator);
		$gpx_creator_text = $dom_gpx->createTextNode('http://thydzik.com');
		$gpx_creator->appendChild($gpx_creator_text);
		
		$gpx_xmlns_xsi = $dom_gpx->createAttribute('xmlns:xsi');
		$gpx->appendChild($gpx_xmlns_xsi);
		$gpx_xmlns_xsi_text = $dom_gpx->createTextNode('http://www.w3.org/2001/XMLSchema-instance');
		$gpx_xmlns_xsi->appendChild($gpx_xmlns_xsi_text);
		
		$gpx_xmlns = $dom_gpx->createAttribute('xmlns');
		$gpx->appendChild($gpx_xmlns);
		$gpx_xmlns_text = $dom_gpx->createTextNode('http://www.topografix.com/GPX/1/0');
		$gpx_xmlns->appendChild($gpx_xmlns_text);
		
		$gpx_xsi_schemaLocation = $dom_gpx->createAttribute('xsi:schemaLocation');
		$gpx->appendChild($gpx_xsi_schemaLocation);
		$gpx_xsi_schemaLocation_text = $dom_gpx->createTextNode('http://www.topografix.com/GPX/1/0 http://www.topografix.com/GPX/1/0/gpx.xsd');
		$gpx_xsi_schemaLocation->appendChild($gpx_xsi_schemaLocation_text);
		
		$gpx_url = $dom_gpx->createElement('url');
		$gpx_url = $gpx->appendChild($gpx_url);
		$gpx_url_text = $dom_gpx->createTextNode($u_parts['dirname']);
		$gpx_url->appendChild($gpx_url_text);
		
		$gpx_time = $dom_gpx->createElement('time');
		$gpx_time = $gpx->appendChild($gpx_time);
		$gpx_time_text = $dom_gpx->createTextNode(utcdate());
		$gpx_time->appendChild($gpx_time_text);
		
		// placemarks
		$names = array();
		foreach ($dom_kml->getElementsByTagName('Placemark') as $placemark) {
			//name
			foreach ($placemark->getElementsByTagName('name') as $name) {
				$name  = $name->nodeValue;
				//check if the key exists
				if (array_key_exists($name, $names)) {
					//increment the value
					++$names[$name];
					$name = $name." ({$names[$name]})";
				} else {
					$names[$name] = 0;
				}
			}
			//description
			foreach ($placemark->getElementsByTagName('description') as $description) {
				$description  = $description->nodeValue;
			}
			foreach ($placemark->getElementsByTagName('Point') as $point) {
				foreach ($point->getElementsByTagName('coordinates') as $coordinates) {
					//add the marker
					$coordinate = $coordinates->nodeValue;
					$coordinate = str_replace(" ", "", $coordinate);//trim white space
					$latlng = explode(",", $coordinate);
					
					if (($lat = $latlng[1]) && ($lng = $latlng[0])) {
						$gpx_wpt = $dom_gpx->createElement('wpt');
						$gpx_wpt = $gpx->appendChild($gpx_wpt);

						$gpx_wpt_lat = $dom_gpx->createAttribute('lat');
						$gpx_wpt->appendChild($gpx_wpt_lat);
						$gpx_wpt_lat_text = $dom_gpx->createTextNode($lat);
						$gpx_wpt_lat->appendChild($gpx_wpt_lat_text);
						
						$gpx_wpt_lon = $dom_gpx->createAttribute('lon');
						$gpx_wpt->appendChild($gpx_wpt_lon);
						$gpx_wpt_lon_text = $dom_gpx->createTextNode($lng);
						$gpx_wpt_lon->appendChild($gpx_wpt_lon_text);
						
						$gpx_time = $dom_gpx->createElement('time');
						$gpx_time = $gpx_wpt->appendChild($gpx_time);
						$gpx_time_text = $dom_gpx->createTextNode(utcdate());
						$gpx_time->appendChild($gpx_time_text);
						
						$gpx_name = $dom_gpx->createElement('name');
						$gpx_name = $gpx_wpt->appendChild($gpx_name);
						$gpx_name_text = $dom_gpx->createTextNode($name);
						$gpx_name->appendChild($gpx_name_text);
						
						$gpx_desc = $dom_gpx->createElement('desc');
						$gpx_desc = $gpx_wpt->appendChild($gpx_desc);
						$gpx_desc_text = $dom_gpx->createTextNode($description);
						$gpx_desc->appendChild($gpx_desc_text);
						
						//$gpx_url = $dom_gpx->createElement('url');
						//$gpx_url = $gpx_wpt->appendChild($gpx_url);
						//$gpx_url_text = $dom_gpx->createTextNode($ref);
						//$gpx_url->appendChild($gpx_url_text);
						
						$gpx_sym = $dom_gpx->createElement('sym');
						$gpx_sym = $gpx_wpt->appendChild($gpx_sym);
						$gpx_sym_text = $dom_gpx->createTextNode('Waypoint');
						$gpx_sym->appendChild($gpx_sym_text);
					}
				}
			}
			foreach ($placemark->getElementsByTagName('LineString') as $lineString) {
				foreach ($lineString->getElementsByTagName('coordinates') as $coordinates) {
					//add the new track
					$gpx_trk = $dom_gpx->createElement('trk');
					$gpx_trk = $gpx->appendChild($gpx_trk);
					
					$gpx_name = $dom_gpx->createElement('name');
					$gpx_name = $gpx_trk->appendChild($gpx_name);
					$gpx_name_text = $dom_gpx->createTextNode($name);
					$gpx_name->appendChild($gpx_name_text);
					
					$gpx_trkseg = $dom_gpx->createElement('trkseg');
					$gpx_trkseg = $gpx_trk->appendChild($gpx_trkseg);
				
					$coordinates = $coordinates->nodeValue;
					$coordinates = preg_split("/[\s\r\n]+/", $coordinates); //split the coords by new line
					foreach ($coordinates as $coordinate) {
						$latlng = explode(",", $coordinate);
						
						if (($lat = $latlng[1]) && ($lng = $latlng[0])) {
							$gpx_trkpt = $dom_gpx->createElement('trkpt');
							$gpx_trkpt = $gpx_trkseg->appendChild($gpx_trkpt);

							$gpx_trkpt_lat = $dom_gpx->createAttribute('lat');
							$gpx_trkpt->appendChild($gpx_trkpt_lat);
							$gpx_trkpt_lat_text = $dom_gpx->createTextNode($lat);
							$gpx_trkpt_lat->appendChild($gpx_trkpt_lat_text);
							
							$gpx_trkpt_lon = $dom_gpx->createAttribute('lon');
							$gpx_trkpt->appendChild($gpx_trkpt_lon);
							$gpx_trkpt_lon_text = $dom_gpx->createTextNode($lng);
							$gpx_trkpt_lon->appendChild($gpx_trkpt_lon_text);
							
							$gpx_time = $dom_gpx->createElement('time');
							$gpx_time = $gpx_trkpt->appendChild($gpx_time);
							$gpx_time_text = $dom_gpx->createTextNode(utcdate());
							$gpx_time->appendChild($gpx_time_text);
						}
					}
				}
			}
		}
		header("Content-Type: text/xml");
		echo $dom_gpx->saveXML();
	}
?>

Let me know if you find any issues.

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