Fix segfault with PHP 5.6 and interned strings

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    PHP         : /usr/bin/php 
    PHP_SAPI    : cli
    PHP_VERSION : 5.6.0
    ZEND_VERSION: 2.6.0
    PHP_OS      : Linux - Linux schrodingerscat.famillecollet.com 3.14.17-100.fc19.x86_64 #1 SMP Thu Aug 14 17:17:26 UTC 2014 x86_64
    ....
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    TEST RESULT SUMMARY
    ---------------------------------------------------------------------
    Exts skipped    :    0
    Exts tested     :  183
    ---------------------------------------------------------------------
    Number of tests :  116               114
    Tests skipped   :    2 (  1.7%) --------
    Tests warned    :    0 (  0.0%) (  0.0%)
    Tests failed    :    0 (  0.0%) (  0.0%)
    Expected fail   :    0 (  0.0%) (  0.0%)
    Tests passed    :  114 ( 98.3%) (100.0%)
    ---------------------------------------------------------------------
    Time taken      :   37 seconds
    =====================================================================