the array is $image, $Lg is the number of the discrete possible values
$n_element=count($image);
$cdf_array=array_fill(0,$Lg,0);
for($i=0;$i<$n_element;$i++){
$cdf_array[$image[$i]-1]=$cdf_array[$image[$i]-1]+1;
}
$cdf_array=array_filter($cdf_array,function($var){return $var!=0;});
$keys=array_keys($cdf_array);
$cdf_min=$cdf_array[$keys[0]];
$not_null_el=count($keys);
for($i=0;$i<$not_null_el;$i++){
$cdf_array[$keys[$i]]=array_sum(array_chunk($cdf_array,$i+1,1)[0]);
}
}