fix #1535 xdebug installation fails when php version is 5.6

php-fpm/Dockerfile

@@ -86,7 +86,11 @@ ARG INSTALL_XDEBUG=false
 
 RUN if [ ${INSTALL_XDEBUG} = true ]; then \
   # Install the xdebug extension
-    pecl install xdebug && \
+  if [ $(php -r "echo PHP_MAJOR_VERSION;") = "5" ]; then \
+    pecl install xdebug-2.5.5; \
+  else \
+    pecl install xdebug; \
+  fi && \
   docker-php-ext-enable xdebug \
 ;fi