Compare this to log1p (which is its inverse).
Also, You may have to use a similar workaraound in case the underlying C library
does not support expm1:
<?php
function expm1($x) {
return ($x>-1.0e-6 && $x<1.0e-6) ? ($x + $x*$x/2) : (exp($x)-1);
}
?>