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<?php
$array = 
    array(
        // base10 => base58
        '3429289555' => '6e31iZ',
        '3368' => '215',
        '74' => '2h',
        '75' => '2i',
        '94' => '2C',
        '88' => '2w',
        '195102' => 'ZZQ',
        '1253576' => '7qDo',
        '177' => '44',
        '193' => '4k',
        '195' => '4n'
    );
 
foreach ($array as $base10 => $base58) {
    echo 'base58_encode(\'', $base10, '\') => ', base58_encode($base10), "\n";
}
 
echo "\n";
foreach ($array as $base10 => $base58) {
    echo 'base58_decode(\'', $base58, '\') => ', base58_decode($base58), "\n";
}

<?php
function base58_decode($num) {
    $alphabet = '123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ';
    $len = strlen($num);
    $decoded = 0;
    $multi = 1;
 
    for ($i = $len - 1; $i >= 0; $i--) {
        $decoded += $multi * strpos($alphabet, $num[$i]);
        $multi = $multi * strlen($alphabet);
    }
 
    return $decoded;
}

<?php
function base58_encode($num) {
    $alphabet = '123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ';
    $base_count = strlen($alphabet);
    $encoded = '';
 
    while ($num >= $base_count) {
        $div = $num / $base_count;
        $mod = ($num - ($base_count * intval($div)));
        $encoded = $alphabet[$mod] . $encoded;
        $num = intval($div);
    }
 
    if ($num) {
        $encoded = $alphabet[$num] . $encoded;
    }
 
    return $encoded;
}

Source
http://darklaunch.com/2009/08/07/base58-encode-and-decode-using-php-with-example-base58-encode-base58-decode