Description:
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Both the current master branch and the extension's 2.4.0 release are affected in php 7.1. 7.0 doesn't seem to have this assertion.

Solr declares that it returns a reference for many functions in the method signatures. However, it returns IS_OBJECT or IS_NULL in many places in php 7 (E.g. uses ZVAL_COPY for the implementation of solr_return_solr_params_object();
 (But does set the is_ref flag in php 5). (I haven't investigated why it declared itself as returning a reference, but it didn't seem to make sense for SolrQuery->addFilterQuery())

Various errors of the below form are seen:

001+ php: /usr/local/php-7.1.0-src/Zend/zend_vm_execute.h:980: ZEND_DO_FCALL_SPEC_RETVAL_UNUSED_HANDLER: Assertion `(call->func->common.fn_flags & 0x4000000) ? (
zval_get_type(&(*(ret))) == 10) : !(zval_get_type(&(*(ret))) == 10)' failed.

(0x4000000 is ZEND_ACC_RETURN_REFERENCE)

A patch is included, and tests passed for php 7.0 and php 7.1 built with --enable-debug. I 

Test script:
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# Build php with --enable-maintainer-zts --enable-debug --enable-libxml

# in another window, run a solr server
sudo docker run -p 8983:8983 --name solr5 omars/solr53
cd solr
phpize
./configure
make clean
make test

Expected result:
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`make test` should have all tests pass in php 7.1 when php is compiled with --enable-debug

Actual result:
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Dozens of tests abort with zend assertion errors