Convert UTM northing/easting to latitude and longitude.

UTMtoLatLon.php

<?php
// Original source from Steven Dutch, https://www.uwgb.edu/dutchs/UsefulData/ConvertUTMNoOZ.HTM
// Rewritten by Thomas Orten

function UTMtoLatLon ($input_lat, $input_long, $utm_zone, $south_of_equator = false) {

  $DatumEqRad = array(6378137.0, 6378137.0, 6378137.0, 6378135.0, 6378160.0, 6378245.0, 6378206.4, 6378388.0, 6378388.0, 6378249.1, 6378206.4, 6377563.4, 6377397.2, 6377276.3);	
	$DatumFlat = array(298.2572236, 298.2572236, 298.2572215, 298.2597208, 298.2497323, 298.2997381, 294.9786982, 296.9993621, 296.9993621, 293.4660167, 294.9786982, 299.3247788, 299.1527052, 300.8021499); 
	$Item = 0; // Default
	$k0 = 0.9996; // scale on central meridian
	$a = $DatumEqRad[$Item]; // equatorial radius, meters. 
	$f = 1/$DatumFlat[$Item]; // polar flattening.
	$b = $a*(1-$f); // polar axis.
	$e = sqrt(1 - $b*$b/$a*$a); // eccentricity
	$drad = pi()/180; // Convert degrees to radians)
	$phi = 0; // latitude (north +, south -), but uses phi in reference
	$lng = 0; // Longitude (e = +, w = -) - can't use long - reserved word
	$lng0 = 0; // longitude of central meridian
	$lngd = 0; // longitude in degrees
	$M = 0; // M requires calculation
	$x = 0; // x coordinate
	$y = 0; // y coordinate
	$k = 1; // local scale

	// Convert UTM Coordinates to Geographic
	$k0 = 0.9996; // scale on central meridian
	$b = $a*(1-$f); // polar axis.
	$e = sqrt(1 - ($b/$a)*($b/$a)); // eccentricity
	$e0 = $e/sqrt(1 - $e*$e); // Called e prime in reference
	$esq = (1 - ($b/$a)*($b/$a)); // e squared for use in expansions
	$e0sq = $e*$e/(1-$e*$e); //  e0 squared - always even powers

	// Easting
	$x = floatval($input_long);
	
	if ($x<160000 || $x>840000){ echo("Outside permissible range of easting values. Results may be unreliable. Use with caution \n"); } 

	// Northing
	$y = floatval($input_lat);

	if ($y<0){ echo("Negative values not allowed. Results may be unreliable. Use with caution \n"); }
	if ($y>10000000){ echo("Northing may not exceed 10,000,000. Results may be unreliable. Use with caution \n"); }
	
	// Zone. Just number!
	$utmz = floatval($utm_zone);

	$zcm = 3 + 6*($utmz-1) - 180; // Central meridian of zone
	$e1 = (1 - sqrt(1 - $e*$e))/(1 + sqrt(1 - $e*$e)); // Called e1 in USGS PP 1395 also
	$M0 = 0; // In case origin other than zero lat - not needed for standard UTM
	$M = $M0 + $y/$k0; // Arc length along standard meridian. 
	
	// IF not south of qeuator, set to true
	if ($south_of_equator){
		$M=$M0+($y-10000000)/$k;
	}
	
	$mu = $M/($a*(1 - $esq*(1/4 + $esq*(3/64 + 5*$esq/256))));
	$phi1 = $mu + $e1*(3/2 - 27*$e1*1/32)*sin(2*$mu) + $e1*$e1*(21/16 -55*$e1*$e1/32)*sin(4*$mu); //Footprint Latitude
	$phi1 = $phi1 + $e1*$e1*$e1*(sin(6*$mu)*151/96 + $e1*sin(8*$mu)*1097/512);
	$C1 = $e0sq*pow(cos($phi1),2);
	$T1 = pow(tan($phi1),2);
	$N1 = $a/sqrt(1-pow($e*sin($phi1),2));
	$R1 = $N1*(1-$e*$e)/(1-pow($e*sin($phi1),2));
	$D = ($x-500000)/($N1*$k0);
	$phi = ($D*$D)*(1/2 - $D*$D*(5 + 3*$T1 + 10*$C1 - 4*$C1*$C1 - 9*$e0sq)/24);
	$phi = $phi + pow($D,6)*(61 + 90*$T1 + 298*$C1 + 45*$T1*$T1 -252*$e0sq - 3*$C1*$C1)/720;
	$phi = $phi1 - ($N1*tan($phi1)/$R1)*$phi;
				
	//Output Latitude
	$lat = floor(1000000*$phi/$drad)/1000000;
		
	//Longitude
	$lng = $D*(1 + $D*$D*((-1 -2*$T1 -$C1)/6 + $D*$D*(5 - 2*$C1 + 28*$T1 - 3*$C1*$C1 +8*$e0sq + 24*$T1*$T1)/120))/cos($phi1);
	$lngd = $zcm+$lng/$drad;
	
	//Output Longitude
	$long = floor(1000000*$lngd)/1000000;

	if ($lat == 0 || $lat == 0) {
		echo "Could not convert $input_lat, $input_long. \n";
	} else {
		echo "Latitude: $lat , Longitude: $long \n";
		return array (
			'lat' => $lat,
			'lng' => $long
		);
	}
}