Description:
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From the documentation for number_format function:

thousands_sep
    Sets the thousands separator.
    Only the first byte of thousands_sep is used. 

This makes it useless for any real world usage. The recommended HTML character for thousands separator is half-space, or   which is 8 characters. Function is only usable for obsolete or archaic systems

http://en.wikipedia.org/wiki/Thin_space
http://en.wikipedia.org/wiki/Thousands_separator#Digit_grouping

Test script:
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echo number_format(123456, 2, '.', ' ');

Expected result:
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123 456.00

Actual result:
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123&456.00