Example failure from https://lists.wikimedia.org/pipermail/wikitech-l/2015-October/083675.html:
1) ApiFormatPhpTest::testGeneralEncoding with data set #7 (array(1.0E+42), 'a:1:{i:0;d:1.0E+42;}', array(1))
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-a:1:{i:0;d:1.0E+42;}
+a:1:{i:0;d:1000000000000000044885712678075916785549312;}
/usr/home/saper/public_html/ybabel/tests/phpunit/includes/api/format/ApiFormatTestBase.php:61
/usr/home/saper/public_html/ybabel/tests/phpunit/MediaWikiTestCase.php:137
2) ApiFormatPhpTest::testGeneralEncoding with data set #30 (array(1.0E+42), 'a:1:{i:0;d:1.0E+42;}', array(2))
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-a:1:{i:0;d:1.0E+42;}
+a:1:{i:0;d:1000000000000000044885712678075916785549312;}
/usr/home/saper/public_html/ybabel/tests/phpunit/includes/api/format/ApiFormatTestBase.php:61
/usr/home/saper/public_html/ybabel/tests/phpunit/MediaWikiTestCase.php:137The cause was running PHP with serialize_precision set to 100.