I went searching for how to calculate the area of a rectangle* on the surface of the earth using PHP and didn’t find one online.  Someone probably has one somewhere, but it was just as fast for me to adapt one as it was to continue searching, so here it is.  Enjoy.

This code is also available on GitHub if you prefer:
https://github.com/prairiewest/PHPRectangleArea

<?php

/*
 * Copyright 2013 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 *
 * PHP code by Todd Trann
 * Adapted from SphericalUtil.java in Google maps source code
 * https://github.com/googlemaps/android-maps-utils/blob/master/library/src/com/google/maps/android/SphericalUtil.java
 *
 */

/*
 * Returns the area of a closed path on Earth in sq km
 * Path is an array of arrays
 * Earth radius is fixed at 6371 km
 */
function computeAreaKm2($path) {
    return abs(computeSignedArea($path,6371));
}

/*
 * Returns the area of a closed path on Earth in acres
 * Path is an array of arrays
 * Earth radius is fixed at 6371 km
 */
function computeAreaAcres($path) {
    return abs(computeSignedArea($path,6371)) * 247.105;
}

/*
 * Returns the signed area of a closed path on a sphere of given radius
 * The computed area uses the same units as the radius squared
 */
function computeSignedArea($path, $radius) {
    $size = count($path);
    if ($size < 3) { return 0; }
    $total = 0;
    $prev = $path[$size-1];
    $prevTanLat = tan((M_PI / 2 - deg2rad($prev['latitude'])) / 2);
    $prevLng = deg2rad($prev['longitude']);
    // For each edge, accumulate the signed area of the polar triangle
    foreach ($path as $point) {
        $tanLat = tan((M_PI / 2 - deg2rad($point['latitude'])) / 2);
        $lng = deg2rad($point['longitude']);
        $total += polarTriangleArea($tanLat, $lng, $prevTanLat, $prevLng);
        $prevTanLat = $tanLat;
        $prevLng = $lng;
    }
    return $total * ($radius * $radius);
}

/*
 * Returns the signed area of a triangle which has North Pole as a vertex
 */
function polarTriangleArea($tan1, $lng1, $tan2, $lng2) {
    $deltaLng = $lng1 - $lng2;
    $t = $tan1 * $tan2;
    return (2 * atan2($t * sin($deltaLng), 1 + $t * cos($deltaLng)));
}

/* ****************************************************
 * CODE BELOW HERE IS NOT NEEDED
 * It is included only to demonstrate use
 */

function makePoint($lat,$lng) {
    $p = array();
    $p['latitude'] = $lat;
    $p['longitude'] = $lng;
    return $p;
}

$p = array(
    makePoint(52.129246,-106.599083),
    makePoint(52.129265,-106.610786),
    makePoint(52.136501,-106.610802),
    makePoint(52.136486,-106.599091)
);

echo "The area is: " . computeAreaAcres($p) . " acres\n";

?>

*Note: more correctly, the area of a quadrangle bounded by the closed path formed using a set of latitude/longitude pairs. But there isn’t much need to be pedantic on the Internet, is there? 😉