The value of mbstring.func_overload is an integer that tells Mbstring which functions it has to override.
For us, randomly, the php engine is behaving like if mbstring.func_overload was equal to 4.
Here is a table of the possible values : http://www.php-editors.com/php_manual/ref.mbstring.html#mbstring.overload

The overload of classic str functions by mb_str functions should not be a problem. But actually it's a problem because :
1/ mb_substr_count and substr_count don't have the same signatures/arguments. I don't know the hell why.
2/ it seems that ereg_replace doesn't complain about a mismatching parenthesis in a regex pattern. But mb_ereg_replace complains.

It is the hoster problem, not ss. To solve it, edit .htaccess file and set: php_value mbstring.func_overload 1.

If you are unable to override the mbstring.func_overload in .htaccess

The suggestion above worked for me:

-------------

It happens there is one error in the pattern of two regex in sapphire/core/SSViewer.php.
Ereg_replace doesn't complain about it, but mb_ereg_replace (and preg_replace too) does. At lines 494 and 496, the last closed parenthesis of the pattern needs to be escaped

,\<\?= +([^\?]*) +\?\>) +%' . '>',

becomes

,\<\?= +([^\?]*) +\?\>\) +%' . '>',

Setting php_value mbstring.func_overload 1 is not good for your server letters (the coding becomes wrong).
The best, is to set .htaccess:

php_value output_buffering On
php_value output_handler mb_output_handler
php_value default_charset UTF-8
php_value mbstring.language Russian(set yours)
php_value mbstring.http_input auto
php_value mbstring.http_output UTF-8
php_value mbstring.internal_encoding UTF-8
php_value mbstring.substitute_character none
php_value mbstring.func_overload 6

Thank you! This post probably saved me a few frustrating hours. I had the same problem and Silverstripe would just show an error message after the installation. I installed Silverstripe 2.4.1 (around August 2010) and the problem still doesn't seem to be fixed - or maybe it only appears in certain server environments. I added the following two backslashes in green in /sapphire/core/SSViewer.php , line 581 and 582 in my version of Silverstripe:

		// i18n sprintf(_t(...),$argument) with entity only (no dots in namespace), meaning the current template filename will be added as a namespace
		$content = ereg_replace('<' . '% +sprintf\(_t\((\'([^\.\']*)\'|"([^\."]*)")(([^)]|\)[^ ]|\) +[^% ])*)\),\<\?= +([^\?]*) +\?\>\) +%' . '>', '<?= sprintf(_t(\''. $path[1] . '.\\2\\3\'\\4),\\6) ?>', $content);
		// i18n sprintf(_t(...),$argument)
		$content = ereg_replace('<' . '% +sprintf\(_t\((\'([^\']*)\'|"([^"]*)")(([^)]|\)[^ ]|\) +[^% ])*)\),\<\?= +([^\?]*) +\?\>\) +%' . '>', '<?= sprintf(_t(\'\\2\\3\'\\4),\\6) ?>', $content);

Cheers!
Anatol

Thanks for posting this Anatol.

I can confirm it's still an issue on some hosts when using version 2.4.5

I tried to copy a site which was previously installed on another server and had this error but patching the SSViewer.php file didn't work on a site which had been installed previously.
The only solution I've found is to delete the files, empty the db, load up a fresh version of SilverStripe 2.4.5, patch the file and install as normal.

Just in case this helps anyone else.

Paul

Thanks for this post Anatol, this is still an issue with 2.4.7 as well and this change in the SS_Viewer.php file sorted it nicely.

Cheers