array = new Array();

array[8]='text';

$.post("file.php", {arr:array}, function(data){alert(data)});

/////////////////////////////////

file.php return print_r($_POST);

/////////////////////////////////

alert -

Array

(

[arr] => Array

(

[0] => undefined

[1] => undefined

[2] => undefined

[3] => undefined

[4] => undefined

[5] => undefined

[6] => undefined

[7] => undefined

[8] => text

)

)