How do I convert these coordinates to google map readable coordinates?

I need to convert coordinates in the following form:

N42-53.9° 
W072-16.2°

Into something that is like the following:

-90.7311
0.346944

A php function would be greatly appreciated – or just a formula would also be nice enough.

Answer

I found an online JS calculator and a PHP solution:

<?php
function DMStoDEC($deg,$min,$sec)
{
// Converts DMS ( Degrees / minutes / seconds ) 
// to decimal format longitude / latitude

    return $deg+((($min*60)+($sec))/3600);
}    

function DECtoDMS($dec)
{
// Converts decimal longitude / latitude to DMS
// ( Degrees / minutes / seconds ) 

// This is the piece of code which may appear to 
// be inefficient, but to avoid issues with floating
// point math we extract the integer part and the float
// part by using a string function.

    $vars = explode(".",$dec);
    $deg = $vars[0];
    $tempma = "0.".$vars[1];

    $tempma = $tempma * 3600;
    $min = floor($tempma / 60);
    $sec = $tempma - ($min*60);

    return array("deg"=>$deg,"min"=>$min,"sec"=>$sec);
}    
?>