<?php
$array = array();
$tmp_string = '';

for( $i=0 ; $i <= 9; $i++ ){
    $tmp_string .= 'x';
    $array[] = trim( $tmp_string );
}
print_r($array);
?>

Array
(
    [0] => x
    [1] => xx
    [2] => xxx
    [3] => xxxx
    [4] => xxxxx
    [5] => xxxxxx
    [6] => xxxxxxx
    [7] => xxxxxxxx
    [8] => xxxxxxxxx
    [9] => xxxxxxxxxx
)