Description:
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In function sapi_cgi_activate, php_cgi_ini_activate_user_config is called with path of the current script and third parameter 'start' which is document_root length - 1. Then, the following code is used:

		ptr = path + start; /* start is the point where doc_root ends! */
		while ((ptr = strchr(ptr, DEFAULT_SLASH)) != NULL) {


However, in general, path is not guaranteed to be residing in DOCUMENT_ROOT. While usually it is the case, there could be rewriting/aliasing scenarios that would call PHP on files outside document root. Or the user can invoke PHP binary by himself too (having DOCUMENT_ROOT set independently, but not intending to use it). 
In such cases, DOCUMENT_ROOT length has no relation to path length, which makes ptr point either to random place inside string if DOCUMENT_ROOT is short, or to random unallocated place in memory if it is longer than current path. 

Reproduce code:
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One can easily reproduce it by setting DOCUMENT_ROOT to very long string and running PHP CGI under valgrind with parameter being some short script path. 

Actual result:
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==15115== Invalid read of size 1
==15115==    at 0x400609A: index (mc_replace_strmem.c:164)
==15115==    by 0x84CE159: php_cgi_ini_activate_user_config (cgi_main.c:716)
==15115==    by 0x84CE40D: sapi_cgi_activate (cgi_main.c:778)
==15115==    by 0x83DA5C1: sapi_activate (SAPI.c:392)
==15115==    by 0x83D08AC: php_request_startup (main.c:1342)
==15115==    by 0x84D04C1: main (cgi_main.c:1961)
==15115==  Address 0x4A65693 is 5 bytes before a block of size 50 alloc'd
==15115==    at 0x4005400: malloc (vg_replace_malloc.c:149)
==15115==    by 0x844B09C: __zend_malloc (zend_alloc.h:81)
==15115==    by 0x844ADC2: _zend_hash_add_or_update (zend_hash.c:247)
==15115==    by 0x84CE0E9: php_cgi_ini_activate_user_config (cgi_main.c:704)
==15115==    by 0x84CE40D: sapi_cgi_activate (cgi_main.c:778)
==15115==    by 0x83DA5C1: sapi_activate (SAPI.c:392)
==15115==    by 0x83D08AC: php_request_startup (main.c:1342)
==15115==    by 0x84D04C1: main (cgi_main.c:1961)