28.17 Using Slice Checksums

As described in Section 4.21, the Slice compilers can optionally generate check­sums of Slice definitions. For slice2php, the ‑‑checksum option causes the compiler to generate code that adds checksums to the global array Ice_sliceChecksums. The checksums are installed automatically when the PHP code is first parsed; no action is required by the application.

In order to verify a server’s checksums, a client could simply compare the two array objects using a comparison operator. However, this is not feasible if it is possible that the server might return a superset of the client’s checksums. A more general solution is to iterate over the local checksums as demonstrated below:

global $Ice_sliceChecksums;
$serverChecksums = ...
foreach($Ice_sliceChecksums as $key => $value)
{
    if(!isset($serverChecksums[$key]))
        // No match found for type id!
    elseif($Ice_sliceChecksums[$key] != $serverChecksums[$key])
        // Checksum mismatch!
}

In this example, the client first verifies that the server’s dictionary contains an entry for each Slice type ID, and then it proceeds to compare the checksums.