Perhaps some one can find useful this function to compute the modular inverse of a integer (extended euclidean algorithm):
function invmod($a,$b) {
$n=$b;
$x=0; $lx=1; $y=1; $ly=0;
while ($b) {
$t=$b;
$q=bcdiv($a,$b,0);
$b=bcmod($a,$b);
$a=$t;
$t=$x; $x=bcsub($lx,bcmod(bcmul($q,$x),$n)); $lx=$t;
$t=$y; $y=bcsub($ly,bcmod(bcmul($q,$y),$n)); $ly=$t;
}
if (bccomp($lx,0) == -1)
$lx=bcadd($lx,$n);
return $lx;
}
// verify
$n="2447995268898324993537772139997802321";
$t="64941057316178801556773346239351236811";
$m="123456789";
$i=invmod($t,$n);
// (t*m)*inv(t) is m
echo bcmod(bcmul(bcmod(bcmul($t,$m),$n),$i),$n) == $m;
