Fix undefined offset in Matomo PHP Tracker #16397

( ! ) Notice: Undefined offset: 3 in vendor/piwik/piwik-php-tracker/PiwikTracker.php on line 1296

This is now happening in Matomo tracker because of https://github.com/matomo-org/matomo/pull/15774/files#diff-1279d666063b65e6d6777f902d11574fR3322-R3330

See https://github.com/matomo-org/matomo-php-tracker/blob/master/MatomoTracker.php#L1302-L1306

The cookie now has a lot less parts and different parts.

As part of this issue we need to

• Assess if this breaks anything in PHP tracker (eg what happens if visitCount etc is no longer existing)
• Adjust the PHP tracker code to understand both type of cookies (older visits would have old cookie structure vs some will have new cookie structure)
• Mention in Matomo 4 developer changelog clearly that people need to upgrade their PHP tracker if they use it to not break anything

@tsteur created https://github.com/matomo-org/matomo-php-tracker/pull/79
Note: doesn't break anything since we don't send those tracking params anymore. Users still need to update. After it's merged, I'll create a PR w/ the changelog info.

Also To do:

• Create issues in all SDKs that have used the PHP code initially such as the C#, possibly Java SDK, and Python