Restored FFI to working order (#28)

* Restored FFI to working order

* Disabled incompatible tests, fixed string conversion

* Use extension_loaded instead of checking PHP version

* Added better checks for FFI

* Do not build FFI deps if no FFI is present

src/AWS/CRT/Internal/FFI.php

@@ -0,0 +1,82 @@
+<?php
+
+namespace AWS\CRT\Internal;
+
+use Exception;
+use RuntimeException;
+
+/**
+ * @internal
+ * Forwards calls on to libaws-crt-ffi via FFI
+ */
+final class FFI {
+    private static $ffi = null;
+    private static $refcount = 0;
+
+    function __construct() {
+        if (is_null(self::$ffi)) {
+            try {
+                $shlib_ext = 'so';
+                $uname_s = php_uname('s');
+                if ($uname_s == 'Darwin') {
+                    $shlib_ext = 'dylib';
+                } else if ($uname_s == 'WINNT') {
+                    $shlib_ext = 'dll';
+                }
+                self::$ffi = \FFI::cdef(
+                    file_get_contents(__DIR__ . "/../../../api.h"),
+                    __DIR__ . "/../../../libaws-crt-ffi." . $shlib_ext);
+                self::init();
+            } catch (Exception $e) {
+                throw new RuntimeException('Exception while initializing CRT via FFI', 0, $e);
+            }
+        }
+        ++self::$refcount;
+    }
+
+    function __destruct() {
+        if (--self::$refcount == 0) {
+            self::clean_up();
+            self::$ffi = null;
+        }
+    }
+
+    /**
+     * Forwards any call made on this object to the FFI function of the
+     * same name with the supplied arguments. Argument type hinting and checking
+     * occurs at the CRT wrapper.
+     */
+    function __call(string $name, $args) {
+        // Expand strings to (string, length)
+        $ffi_args =[];
+        foreach ($args as $arg) {
+            if (is_string($arg)) {
+                $len = strlen($arg);
+                if ($len > 0) {
+                    $uint8_t = \FFI::type('uint8_t');
+                    $uint8_array = \FFI::arrayType($uint8_t, [$len]);
+                    $buf = \FFI::new($uint8_array);
+                    \FFI::memcpy($buf, $arg, $len);
+                    $ffi_args [] = $buf;
+                    $ffi_args [] = $len;
+                } else {
+                    $ffi_args [] = null;
+                    $ffi_args [] = 0;
+                }
+            } else if (is_resource($arg)) {
+                throw new RuntimeException("Resource types are not supported for FFI");
+            } else {
+                $ffi_args []= $arg;
+            }
+        }
+        return call_user_func_array([self::$ffi, $name], $ffi_args);
+    }
+
+    private static function init() {
+        return self::$ffi->aws_crt_init();
+    }
+
+    private static function clean_up() {
+        return self::$ffi->aws_crt_clean_up();
+    }
+}