<?php

$array = array(array('bla1' => 'bla1'), array('bla2' => 'bla2'),
    array('bla3' => 'bla3'), array('bla4' => 'bla4'));

$b = array(array('bla2.5' => 'bla2.5'));

array_splice($array, 2, 0, $b);

print_r($array);

Array
(
    [0] => Array
        (
            [bla1] => bla1
        )

    [1] => Array
        (
            [bla2] => bla2
        )

    [2] => Array
        (
            [bla2.5] => bla2.5
        )

    [3] => Array
        (
            [bla3] => bla3
        )

    [4] => Array
        (
            [bla4] => bla4
        )

)