Example of creating a C structure with a function pointer via FFI, setting a PHP callback to that function pointer and calling it via FFI, and calling other PHP callbacks from within it

ffi_callback.php

<?php

use FFI\CData;

$ffi = FFI::cdef(
    <<<CDEF
    typedef void (*func)(void (*callable1)(void), const char *(*callable2)(const char *));
    struct tmp {
        func fp;
    };
    CDEF
);

$tmp = $ffi->new('struct tmp');
$tmp->fp = function (callable $callable1, callable $callable2): void {
    $callable1();
    var_dump($callable2('foo'));
};

$return = FFI::new('char[' . strlen('foo') + 1 . ']', false);
($tmp->fp)(function (): void {}, function (string $s) use ($return): CData {
    FFI::memcpy($return, $s, strlen($s));
    $return[strlen($s)] = "\0";
    return $return;
});
FFI::free($return);