ereg_replace to encode numbers

[alirezaok]

as u know this code replase all numbers in variable to "5"

$a="P 3 R 7 c G U 9 Z H l u Y W";
echo ereg_replace("[0-9]","5",$a);

but i want to all number be encode with base64_encode()
in other mean in this example the ouput must:

P Mw== R Nw== c G U OQ== Z H l u Y W

any idea?

[Installer]

$a = 'P 3 R 7 c G U 9 Z H l u Y W';
$str = '';
for ($i = 0; $i < strlen($a); $i++) {
    if (ctype_digit($a{$i})) {
        $str .= base64_encode($a{$i});
    } else {
        $str .= $a{$i};
    }
}

[alirezaok]

Thanks Installer.

[bradgrafelman]

Or...

$a="P 3 R 7 c G U 9 Z H l u Y W";
$a = preg_replace('/([0-9])/e', 'base64_encode("$1")', $a);