Description:
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number_format will only accept a single character for the thousands 
separator and deliberately shortens longer input to one character.

However, for some charsets (ru_RU.UTF8 was the one I discovered this on) 
the thousands separator as returned by localeconv is more than one 
character.
Normally, this wouldn't be a problem, however, it seems when PHP tries 
to shorten this particular value to one character, it creates an invalid 
character and returns something like demonstrated in the reproduce code.

Reproduce code:
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setlocale( L_ALL, 'ru_RU.UTF8' );
$locale = localeconv();	
echo number_format( 9999, 0, $locale['decimal_point'], $locale['thousands_sep'] );

Expected result:
----------------
9 999

Actual result:
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9?999