HTML5 Geolocation API'sini kullanmanız gerekir.

PHP:
<?php
session_start();
if (!isset($_SESSION['latitude']) && !isset($_SESSION['longitude'])) {
?>
<script>
navigator.geolocation.getCurrentPosition(success);
function success(position) {
var latitude = position.coords.latitude;
var longitude = position.coords.longitude;
window.location = "index.php?latitude=" + latitude + "&longitude=" + longitude;
}
</script>
<?php
} else {
$latitude = $_SESSION['latitude'];
$longitude = $_SESSION['longitude'];
echo "Latitude: " . $latitude . "<br>Longitude: " . $longitude;
}

if (isset($_GET['latitude']) && isset($_GET['longitude'])) {
$_SESSION['latitude'] = $_GET['latitude'];
$_SESSION['longitude'] = $_GET['longitude'];
}
?>

JS

<script>
window.onload = function() {
navigator.geolocation.getCurrentPosition(success);
function success(position) {
var latitude = position.coords.latitude;
var longitude = position.coords.longitude;
console.log("Latitude: " + latitude);
console.log("Longitude: " + longitude);
}
}
</script>