fixed #30 - wrong use of variable variables on big endian

[marc-mabe]

fixes #30

@remicollet please check if that fixes the issue - thanks

[remicollet]

@marc-mabe sorry for not being able to test this PR yet (terribly busy with release week) Should be able to give you some feedback before end of week

[remicollet]

Still failing on bigendian

+ php /usr/bin/phpunit
PHPUnit 5.7.23 by Sebastian Bergmann and contributors.
SSSSSSSSSSS.............SSSSSSSSSSSS.........................S.  63 / 185 ( 34%)
.....................................................F......... 126 / 185 ( 68%)
...........................................................     185 / 185 (100%)
Time: 88 ms, Memory: 6.00MB
There was 1 failure:
1) ZendTest\Serializer\Adapter\PythonPickleUnserializeTest::testUnserializeLong4BigInt
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'-6944656592455360768'
+'44930029223649183'
/builddir/build/BUILD/zend-serializer-ff74ea020f5f90866eb28365327e9bc765a61a6e/test/Adapter/PythonPickleUnserializeTest.php:164
FAILURES!

See Fedora scratch build for various archictectures: https://koji.fedoraproject.org/koji/taskinfo?taskID=22988940

Failing on ppc64 and s390x (both bigendian)

[marc-mabe]

@remicollet I'm very happy to have you testing this :+1: -> Please try again

[remicollet]

New Fedora scratch build https://koji.fedoraproject.org/koji/taskinfo?taskID=23007185

The test suite passes on all arch \o/

Thanks