php - How to create the array with key and value every time? -

i want create array such $animal = array("a" => "horse","b" => "fish").
constraint condition create 1 element (key , value) in array 1 time,that say
in first time create key "a" , value "horse" ,to make $animal = array("a" => "horse"),
in second time create key "b" , value "fish",to make $animal = array("a" => "horse","b" => "fish").

i can create array("horse","fish") in 2 times, in first time make array array("horse"),
in second time make array array("horse","fish").

<?php         $animal = array();         $x2 = "horse";         $x4 = "fish";         $animal[] = $x2;         $animal[] = $x4;         print_r($animal);     ?>     

how create array("a" => "horse","b" => "fish") in same way?

<?php         $animal = array();         $x1 = "a";         $x2 = "horse";         $x3 = "b";         $x4 = "fish";         array_keys($animal[]) = $x1;         array_values($animal[]) = $x2;         array_keys($animal[]) = $x3;         array_values($animal[]) = $x4;         print_r($animal); ?> 

how fix code job?

when using [] push elements array, can specify key, if no key specified, default values used, therefor:

<?php         $animal = array();         $x1 = "a";         $x2 = "horse";         $x3 = "b";         $x4 = "fish";         $animal[$x1] = $x2;         $animal[$x3] = $x4;       print_r($animal); ?>