Disable SDK from autoloading during composer execution.

composer.json

@@ -207,5 +207,10 @@
                 "OpenTelemetry\\Tests\\Integration\\SDK\\Common\\Configuration\\TestEnvSourceProvider"
             ]
         }
+    },
+    "scripts": {
+        "test-psr3": [
+            "OpenTelemetry\\Tests\\Integration\\Composer\\Psr3Compatability::run"
+        ]
     }
 }

src/SDK/Common/Util/ComposerHandler.php

@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OpenTelemetry\SDK\Common\Util;
+
+use function basename;
+use function getenv;
+use function in_array;
+
+final class ComposerHandler
+{
+    public static function isRunning(): bool
+    {
+        /**
+         * This is set when composer is running a script; eg:
+         * composer run-script test-psr3
+         *
+         * However, it is not set in the following case:
+         * composer test-psr3
+         */
+        if (getenv('COMPOSER_DEV_MODE')) {
+            return true;
+        }
+
+        if (
+            ($entrypoint = $_SERVER['argv'][0] ?? '') === getenv('COMPOSER_BINARY')
+            || in_array(basename($entrypoint), ['composer', 'composer.phar'], true)
+        ) {
+            return true;
+        }
+
+        return false;
+    }
+}

src/SDK/_autoload.php

@@ -2,4 +2,17 @@
 
 declare(strict_types=1);
 
-\OpenTelemetry\SDK\SdkAutoloader::autoload();
+/**
+ * If OTEL_PHP_AUTOLOAD_ENABLED=true, there may be compatability issues
+ * if the version of PSR-3 installed in ./vendor conflicts with that of the
+ * packaged composer PSR-3 library.
+ *
+ * If COMPOSER_DEV_MODE is present, then we can assume that a composer script
+ * is running, and we can prevent the PSR-3 compatability issues by disabling
+ * the SDK from activating.
+ *
+ * @see https://github.com/open-telemetry/opentelemetry-php/issues/1673
+ */
+if (\OpenTelemetry\SDK\Common\Util\ComposerHandler::isRunning() === false) {
+    \OpenTelemetry\SDK\SdkAutoloader::autoload();
+}

tests/Integration/Composer/Psr3Compatability.php

@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace OpenTelemetry\Tests\Integration\Composer;
+
+use Composer\Script\Event;
+
+final class Psr3Compatability
+{
+    public static function run(Event $event): void
+    {
+        require_once $event->getComposer()->getConfig()->get('vendor-dir') . '/autoload.php';
+    }
+}