salesforce 15 digit to 18 digit id. Function to calculate the checksum.

salesForceIdChecksum.func.php

<?php

function salesForceIdChecksum($id)
{
    $map = implode('',array_merge(range('A', 'Z'), range(0, 9)));
    $checksum = '';

    foreach (str_split($id, 5) as $chunk) {
        $checksum .= substr($map, bindec( strrev(array_reduce(str_split($chunk, 1),function($carry, $item){
            $carry .= (!is_numeric($item) && $item == strtoupper($item)) ? '1' : '0';
            return $carry;
        },''))),1);
    }

    return $checksum;
}

// example
//$_15ID = '001b000000dA3YY';
//$_18ID = $_15ID. salesForceIdChecksum($_15ID);