Check if all dependencies are installed (#639)

* Added check if all dependencies are installed and config.php/config-dist.php exists

* Also check for php modules

* Uncomment code that was accidentally disabled for debug purposes

helpers/PrerequisiteChecker.php

@@ -0,0 +1,50 @@
+<?php
+
+class ERequirementNotMet extends Exception {
+}
+
+
+const REQUIRED_PHP_EXTENSIONS = array("fileinfo", "pdo_sqlite", "gd");
+
+class PrerequisiteChecker {
+    
+    public function checkRequirements() {
+        self::checkForConfigFile();
+        self::checkForConfigDistFile();
+        self::checkForComposer();
+        self::checkForYarn();
+        self::checkForPhpExtensions();
+    }
+    
+    
+    private function checkForConfigFile() {
+        if (!file_exists(__DIR__ . "/../data/config.php"))
+            throw new ERequirementNotMet("/data/config.php not found. Have you copied config-dist.php to the data directory and renamed it to config.php?");
+    }
+
+    private function checkForConfigDistFile() {
+        if (!file_exists(__DIR__ . "/../config-dist.php"))
+            throw new ERequirementNotMet("config-dist.php not found. Please do not remove this file.");
+    }
+
+    private function checkForComposer() {
+        if (!file_exists(__DIR__ . "/../vendor/autoload.php"))
+            throw new ERequirementNotMet("/vendor/autoload.php not found. Have you run Composer?");
+    }
+
+    private function checkForYarn() {
+        if (!file_exists(__DIR__ . "/../public/node_modules"))
+            throw new ERequirementNotMet("/public/node_modules not found. Have you run Yarn?");
+    }
+
+    private function checkForPhpExtensions() {
+        $loadedExtensions = get_loaded_extensions();
+        foreach (REQUIRED_PHP_EXTENSIONS as $extension) {
+            if (!in_array($extension, $loadedExtensions))
+                throw new ERequirementNotMet("PHP module '{$extension}' not installed, but required.");
+        }
+    }
+}
+
+
+?>