The function used to convert 64bit Steam ID to 32bit and the other way around.

steamid_conversion.php

<?php

function convert_id($id)
{
    if (strlen($id) === 17)
    {
        $converted = substr($id, 3) - 61197960265728;
    }
    else
    {
        $converted = '765'.($id + 61197960265728);
    }

    return (string) $converted;
}

$sixtyfour = '76561197992765754';
$thirtytwo = '32500026';

var_dump(convert_id($thirtytwo) === $sixtyfour); // bool(true)
var_dump(convert_id($sixtyfour) === $thirtytwo); // bool(true)