如何构建尽可能多的`if then else`条件作为数组的`count`？

The problem I have is the following: I have a variable number of array items, so I can not have a fix number of if then else. In any case, it can not exceed 50 items.

This is the main-standard body. How to build as many if then else conditions as the count of an array on the fly ?

$number = count($array);

    if (($a > $b) && ($a <= $b)) {
        }   else

You are trying to use Switch in fact, please read : http://php.net/manual/fr/control-structures.switch.php

switch ($number) {
    case 0:
        echo "a";
        break;
    case 1:
        echo "b";
        break;
    case 2:
        echo "c";
        break;
}

This is a bit of a guess, as we don't precisely know what you are trying to achieve, but to answer your question "How to build as many if then else conditions as the count of an array on the fly?":

<?php
$array = array(1,2,5,8,10,15);
$number = count($array);
$b = 7;   // we don't know yet what $a and $b could be

for ($i=0;$i<$number;$i++) {
   if($array[$i]>$b) {   // what ever condition you want here
       // do smth
   } else {
       // do smth else or nothing
   }
}
?>