add test and adjust rule error message

src/Composer/DependencyResolver/Rule.php

@@ -207,7 +207,9 @@ class Rule
                         } elseif ($targetName === 'hhvm') {
                             $text .= ' -> you are running this with PHP and not HHVM.';
                         } else {
-                            $text .= ' -> your PHP version ('. phpversion() .') or value of "config.platform.php" in composer.json does not satisfy that requirement.';
+                            $available = $pool->whatProvides($targetName);
+                            $version = count($available) ? $available[0]->getPrettyVersion() : phpversion();
+                            $text .= ' -> your PHP version or "config.platform.php" value ('.$version.') does not satisfy that requirement.';
                         }
                     } elseif (0 === strpos($targetName, 'ext-')) {
                         // handle php extensions

tests/Composer/Test/Fixtures/installer/github-issues-4319.test

@@ -0,0 +1,45 @@
+--TEST--
+
+See Github issue #4319 ( github.com/composer/composer/issues/4319 ).
+
+Present a clear error message when config.platform.php version results in a conflict rule.
+
+--CONDITION--
+!defined('HHVM_VERSION')
+
+--COMPOSER--
+{
+    "repositories": [
+        {
+            "type": "package",
+            "package": [
+                { "name": "a", "version": "1.0.0", "require": { "php": "5.5" } }
+            ]
+        }
+    ],
+    "require": {
+        "a": "~1.0"
+    },
+    "config": {
+        "platform": {
+            "php": "5.3"
+        }
+    }
+}
+
+--RUN--
+install
+
+--EXPECT-OUTPUT--
+Loading composer repositories with package information
+Installing dependencies (including require-dev)
+Your requirements could not be resolved to an installable set of packages.
+
+  Problem 1
+    - Installation request for a ~1.0 -> satisfiable by a[1.0.0].
+    - a 1.0.0 requires php 5.5 -> your PHP version or "config.platform.php" value (5.3) does not satisfy that requirement.
+
+--EXPECT--
+
+--EXPECT-EXIT-CODE--
+2