Description:
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From manual page: http://www.php.net/function.number-format
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The final three parameters for the number_format function have a default but when only three of the parameters are filled in, the fourth has become mandatory and it returns null (no fatal error)

Test script:
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<?php

var_dump(number_format((float) 1.6, 1, '.'));

Expected result:
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This should output the string 1.6

Actual result:
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This outputs NULL