php 5.6 needs an older version of xdebug

php-unit-test/5.6/Dockerfile

@@ -13,7 +13,7 @@ LABEL org.label-schema.build-date=$BUILD_DATE \
       maintainer="Alan Nelson <alan.nelson@nebraska.edu>"
 
 RUN apk add --no-cache bash nodejs nodejs-npm $PHPIZE_DEPS \
-    && pecl install xdebug \
+    && pecl install xdebug-2.5.0 \
     && docker-php-ext-enable xdebug
 RUN curl -o /usr/local/bin/composer https://getcomposer.org/composer.phar \
   && chmod 755 /usr/local/bin/composer