verbose error message on bin/detect failure

Author: dzuelke

CHANGELOG.md

@@ -1,5 +1,11 @@
 # heroku-buildpack-php CHANGELOG
 
+## v141 (2018-08-??)
+
+### CHG
+
+- Verbose error messasge on `bin/detect` failure [David Zuelke]
+
 ## v140 (2018-07-25)
 
 ### CHG

bin/detect

@@ -1,7 +1,29 @@
 #!/usr/bin/env bash
 
+# convenience functions
+source "$(cd $(dirname $0); cd ..; pwd)"/bin/util/common.sh
+
 if [[ -f "$1/composer.json" || -f "$1/index.php" ]]; then
 	echo "PHP" && exit 0
 else
-	exit 1
+	error <<-EOF
+		Application not supported by this buildpack!
+		
+		The 'heroku/php' buildpack is set on this application, but was
+		unable to detect a PHP codebase.
+		
+		A PHP app on Heroku requires a 'composer.json' at the root of
+		the directory structure, or an 'index.php' for legacy behavior.
+		
+		If you are trying to deploy a PHP application, ensure that one
+		of these files is present at the top level directory.
+		
+		If you are trying to deploy an application written in another
+		language, you need to change the list of buildpacks set on your
+		Heroku app using the 'heroku buildpacks' command.
+		
+		For more information, refer to the following documentation:
+		https://devcenter.heroku.com/articles/buildpacks
+		https://devcenter.heroku.com/articles/php-support#activation
+	EOF
 fi