Calculate poisson probability percentage in php

[runnerjp]

 know in excel to Calculate poisson probability percentage i just use =POISSON( 2, 3.55, FALSE)

but is there a similar thing I can use in php as i have found http://php.net/manual/en/function.stats-cdf-poisson.php but it has no documentation to use it

[requinix]

I'm going to copy/paste from the source:

/**********************************************************************
       Cumulative Distribution Function POIsson distribution
                              Function
     Calculates any one parameter of the Poisson
     distribution given values for the others.
                              Arguments
     WHICH --> Integer indicating which  argument
               value is to be calculated from the others.
               Legal range: 1..3
               iwhich = 1 : Calculate P and Q from S and XLAM
               iwhich = 2 : Calculate A from P,Q and XLAM
               iwhich = 3 : Calculate XLAM from P,Q and S
        P <--> The cumulation from 0 to S of the poisson density.
               Input range: [0,1].
        Q <--> 1-P.
               Input range: (0, 1].
               P + Q = 1.0.
        S <--> Upper limit of cumulation of the Poisson.
               Input range: [0, +infinity).
               Search range: [0,1E100]
     XLAM <--> Mean of the Poisson distribution.
               Input range: [0, +infinity).
               Search range: [0,1E100]
     STATUS <-- 0 if calculation completed correctly
               -I if input parameter number I is out of range
                1 if answer appears to be lower than lowest
                  search bound
                2 if answer appears to be higher than greatest
                  search bound
                3 if P + Q .ne. 1
     BOUND <-- Undefined if STATUS is 0
               Bound exceeded by parameter number I if STATUS
               is negative.
               Lower search bound if STATUS is 1.
               Upper search bound if STATUS is 2.
                              Method
     Formula   26.4.21  of   Abramowitz  and   Stegun,   Handbook  of
     Mathematical Functions (1966) is used  to reduce the computation
     of  the cumulative distribution function to that  of computing a
     chi-square, hence an incomplete gamma function.
     Cumulative  distribution function  (P) is  calculated  directly.
     Computation of other parameters involve a seach for a value that
     produces  the desired value of  P.   The  search relies  on  the
     monotinicity of P with the other parameter.
**********************************************************************/

If $which == 1,

- $par1 is the S value

- $par2 is the XLAM value

If $which == 2,

- $par1 is the P value (and Q is 1-P)

- $par2 is the XLAM value

If $which == 3,

- $par1 is the S value

- $par2 is the P value (and Q is 1-P)