Description:
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From manual page: http://www.php.net/function.stats-cdf-beta
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The return value described in the manual is that of a C routine that is used inside the C code that implements the PHP-callable STATS function.  Within that wrapper code, the desired numerical value is returned by reference, then picked up and made the return value of the PHP-callable function.  (In the case of a parameter-out-of-range error, FALSE is returned.)

For all STATS cdf functions, the desired value is specified by a $which parameter at the end of the parameter list.  A direct-cdf value (the cumulative probability $p corresponding to a position $x) is returned when the function is called with $x as the first parameter and a $which value of 1 as the last parameter.  An inverse-cdf value (the position $x corresponding to a cumulative probability $p) is returned when the function is called with $p as the first parameter and a $which value of 2 as the last parameter.  The second and third parameters of stats_cdf_beta are the distribution-defining $alpha and $beta parameters.  

The other stats_cdf_* functions use the same pattern, except that more or fewer distribution-defining parameters may be used depending on what is appropriate for that function.