Description:
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Documentation reads

round -- Rounds a float
Description
float round ( float val [, int precision] )

Returns the rounded value of val to specified precision (number of digits after the decimal point). precision can also be negative or zero (default). 

Should read:

Returns the rounded value of val's floating point representation in memory to specified precision (number of digits after the decimal point). 

WARNING: This function may not return expected results for floating point numbers in memory whose string representation ends in a 5. eg:

round (7.875 , 1);

returns '7.88'

$a = 45;
$b = $a * .175;
round ($b , 1);

May return either '7.88' or '7.87'. 

See http://www.php.net/manual/en/language.types.float.php

If you would like to know more about "floats" and what IEEE
754 is read this:
http://docs.sun.com/source/806-3568/ncg_goldberg.html
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