feat(install): Start of env var inject script

Start of a script that will inject environment variable
config values into a newrelic.ini config file.

agent/newrelic-install-inject-envvars.php

@@ -0,0 +1,75 @@
+<?PHP
+/*
+ * Copyright 2024 New Relic Corporation. All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/* Purpose: Read a fresh newrelic.ini file and inject any values
+ * from environment variables.
+ * 
+ * Not designed to work on a modified newrelic.ini and script
+ * is not able to detect this scenario.
+ * 
+ * The mapping from INI directive name to corresponding 
+ * environment variable name must be injected before using
+ * in place of the comment below containing INI_ENVVAR_MAP
+ *
+ */
+function failure(string $msg) {
+    die(__FILE__ . " : ERROR : " . $msg . "\n");
+}
+
+function generate_regex(string $ini_name) {
+    $ini_name_esc = str_replace(".", "\.", $ini_name);
+    return "/^\s*;*\s*" . $ini_name_esc . "\s*=.*/im";
+}
+
+/* requires PHP 7+ */
+ if (version_compare(PHP_VERSION, '7.0', '<')) {
+    failure("requires PHP >= 7.0\n");
+}
+
+/* Mapping from INI name to environment variable name
+ * Expected form:
+ *    define('INI_ENVVAR_MAP', array(....));
+ */
+require "newrelic-php-cfg-mappings.php";
+
+/* Verify that ini/envvar mapping was injected and is defined */
+if (!defined('INI_ENVVAR_MAP')) {
+    failure("INI/ENVVAR mapping was not detected - cannot proceed!");
+}
+
+/* Verify input INI file exists */
+if (2 != $argc) {
+    failure("Must supply INI file name as first argument!");
+}
+
+$ini_filename = $argv[1];
+if (!file_exists($ini_filename)) {
+    failure("INI file \"$ini_filename\" does not exist!");
+}
+
+$data = file_get_contents(($ini_filename));
+if (!$data) {
+    failure("Could not read INI file \"$ini_filename\"!");
+}
+
+$pattern = array();
+$replace = array();
+
+/* Go through all INI keys and see if the environment variable is defined */
+foreach (INI_ENVVAR_MAP as $ini_name => $env_name) {
+    array_push($pattern, generate_regex($ini_name));
+    array_push($replace, "$ini_name=$env_name");
+}
+
+$data = preg_replace($pattern, $replace, $data);
+
+$fh = fopen($ini_filename, "w");
+if (!$fh) {
+    failure("Unable to write out modified INI file $ini_filename");
+}
+fwrite($fh, $data);
+fclose($fh);
+