Description:
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Tested with wordpress and apache 2.4 / php-fpm

Define('ABSPATH','mypath') in an auto_prepend_file

When wp-load.php do "define( 'ABSPATH', dirname(__FILE__) . '/' );", my personnal ABSPATH is overwrite with that new ABSPATH.

if i put "get_defined_constants();" in wp-load.php just before "define( 'ABSPATH', dirname(__FILE__) . '/' );", ABSPATH is not overwrite.

Tip:
if i put "get_defined_constants();" in the auto_prepend_file just after my first define,  ABSPATH is  overwrite when wp-load.php do the define function.

if i put "get_defined_constants();" in a file include between the auto_prend_file and wp-load.php (like index.php or wp-blog-header.php)  ABSPATH is  overwrite when wp-load.php do the define function.

Expected result:
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Normally (in php 5.4, php 5.6), a file cannot redefine a constante defined in an auto_prepend_file. 



Actual result:
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Constantes defined in an auto_prepend_file can be redefined