Fonction PHP permettant la conversion de Lambert93 à WGS84 (selon le fichier fourni par l'IGN).

gistfile1.php

<?php
function lambert93ToWgs84($x, $y){
	$x = number_format($x, 10, '.', '');
	$y = number_format($y, 10, '.', '');
	$b6  = 6378137.0000;
	$b7  = 298.257222101;
	$b8  = 1/$b7;
	$b9  = 2*$b8-$b8*$b8;
	$b10 = sqrt($b9);
	$b13 = 3.000000000;
	$b14 = 700000.0000;
	$b15 = 12655612.0499;
	$b16 = 0.7256077650532670;
	$b17 = 11754255.426096;
	$delx = $x - $b14;
	$dely = $y - $b15;
	$gamma = atan( -($delx) / $dely );
	$r = sqrt(($delx*$delx)+($dely*$dely));
	$latiso = log($b17/$r)/$b16;
	$sinphiit0 = tanh($latiso+$b10*atanh($b10*sin(1)));
	$sinphiit1 = tanh($latiso+$b10*atanh($b10*$sinphiit0));
	$sinphiit2 = tanh($latiso+$b10*atanh($b10*$sinphiit1));
	$sinphiit3 = tanh($latiso+$b10*atanh($b10*$sinphiit2));
	$sinphiit4 = tanh($latiso+$b10*atanh($b10*$sinphiit3));
	$sinphiit5 = tanh($latiso+$b10*atanh($b10*$sinphiit4));
	$sinphiit6 = tanh($latiso+$b10*atanh($b10*$sinphiit5));
	$longrad = $gamma/$b16+$b13/180*pi();
	$latrad = asin($sinphiit6);
	$long = ($longrad/pi()*180);
	$lat  = ($latrad/pi()*180);
	
	return array(
		'lambert93' => array(
			'x' => $x,
			'y' => $y
		),
		'wgs84' => array(
			'lat' => $lat,
			'long' => $long
		)
	);
}

// Exemple d'utilisation :
$x = 955502.7409000024;
$y = 6225307.0799999982;
print_r(lambert93ToWgs84($x,$y));

/* Résultat en sortie :
Array
(
    [lambert93] => Array
        (
            [x] => 955502.7409000024
            [y] => 6225307.0799999982
        )

    [wgs84] => Array
        (
            [lat] => 43.081387131355
            [long] => 6.1358573938246
        )
)
*/

Merci pour cette version @Lulucmy !

Si vous ne voulez pas importer toutes les fonctions qui sont dans le module math, vous pouvez rajouter cet import à la place du from math import * :

from math import sqrt, atan, log, tanh, atanh, sin, pi, asin