edward@vds1:~$ cat /home/edward/test.static.php
<?php

test(1,2);
test(3,2);
test(1,5);
test(90,15);

function test($a,$b) {
static $foo = array();
$foo[$a] = $b;
print_r($foo);
}

?>
edward@vds1:~$ php /home/edward/test.static.php
Array
(
    [1] => 2
)
Array
(
    [1] => 2
    [3] => 2
)
Array
(
    [1] => 5
    [3] => 2
)
Array
(
    [1] => 5
    [3] => 2
    [90] => 15
)