minor #10800 using open_basedir will disable the realpath cache (wbob)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #10800).

Discussion
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using open_basedir will disable the realpath cache

as of PHP 7.3.0 this is unresolved, setting an open_basedir directory will disable realpath. This is slightly underdocumented (on php.net ini.core only in the comments). See https://github.com/php/php-src/blob/PHP-7.3.0/main/main.c#L1800 and https://bugs.php.net/bug.php?id=52312

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Commits
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fe0f71d using open_basedir will disable the realpath cache

Author: javiereguiluz

performance.rst

@@ -179,6 +179,8 @@ such as Symfony projects, should use at least these values:
     ; save the results for 10 minutes (600 seconds)
     realpath_cache_ttl=600
 
+Note: using ``open_basedir`` will disable the realpath Cache.
+
 .. _performance-optimize-composer-autoloader:
 
 Optimize Composer Autoloader