frankenphp/docs/known-issues.md

Known Issues

Fibers

Calling PHP functions and language constructs that themselves call cgo in Fibers is known to cause crashes.

This issue is being worked on by the Go project.

In the meantime, one solution is not to use constructs (like echo) and functions (like header()) that delegate to Go from inside Fibers.

This code will likely crash because it uses echo in the Fiber:

$fiber = new Fiber(function() {
    echo 'In the Fiber'.PHP_EOL;
    echo 'Still inside'.PHP_EOL;
});
$fiber->start();

Instead, return the value from the Fiber and use it outside:

$fiber = new Fiber(function() {
    Fiber::suspend('In the Fiber'.PHP_EOL));
    Fiber::suspend('Still inside'.PHP_EOL));
});
echo $fiber->start();
echo $fiber->resume();
$fiber->resume();

Unsupported PHP Extensions

The following extensions are known not to be compatible with FrankenPHP:

Name	Reason	Alternatives
imap	Not thread-safe	javanile/php-imap2, webklex/php-imap

get_browser

The get_browser function seems to have a bad performance after a while. A workaround is to cache (e.g. with APCU) the results per User Agent, as they are static anyway.