Description:
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In documentation i can read :  Only the first character of thousands_sep  is used. For example, if you use foo as thousands_sep on the number 1000, number_format() will return 1f000.

But in my example when i use "Long" parameter it ignored and some default value used.

I test it on versions 4.3.6, 5.1.0b3

Reproduce code:
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<?php
echo number_format(123456789,0, '.', '--');
?>

Expected result:
----------------
123-456-789

Actual result:
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123,456,789