Add is_countable under PHP 7.3

LICENSE

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+Copyright (c) 2018 Fabien Potencier
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is furnished
+to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

README.md

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+Symfony Polyfill / Php73
+========================
+
+This component provides functions added to PHP 7.3 core:
+
+- [`is_countable`](https://php.net/is_countable)
+
+More information can be found in the
+[main Polyfill README](https://github.com/symfony/polyfill/blob/master/README.md).
+
+License
+=======
+
+This library is released under the [MIT license](LICENSE).

bootstrap.php

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+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+if (PHP_VERSION_ID < 70300) {
+    if (!function_exists('is_countable')) {
+        function is_countable($var) {
+            return is_array($var) || $var instanceof Countable;
+        }
+    }
+}

composer.json

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+{
+    "name": "symfony/polyfill-php73",
+    "type": "library",
+    "description": "Symfony polyfill backporting some PHP 7.3+ features to lower PHP versions",
+    "keywords": ["polyfill", "shim", "compatibility", "portable"],
+    "homepage": "https://symfony.com",
+    "license": "MIT",
+    "authors": [
+        {
+            "name": "Nicolas Grekas",
+            "email": "p@tchwork.com"
+        },
+        {
+            "name": "Symfony Community",
+            "homepage": "https://symfony.com/contributors"
+        }
+    ],
+    "require": {
+        "php": ">=5.3.3"
+    },
+    "autoload": {
+        "psr-4": { "Symfony\\Polyfill\\Php73\\": "" },
+        "files": [ "bootstrap.php" ]
+    },
+    "minimum-stability": "dev",
+    "extra": {
+        "branch-alias": {
+            "dev-master": "1.8-dev"
+        }
+    }
+}