$aaa[]='a';

<?php
//$aaa=array();
$aaa[]='a';
$aaa[]='b';
$aaa[]='c';
var_dump($aaa);

array (size=3)
  0 => string 'a' (length=1)
  1 => string 'b' (length=1)
  2 => string 'c' (length=1)