<?php

$string = 'db1
catA
cat1a1
prod1:a0
prod2:a0
prod3:a0
cat2a2
prod1:a1
prod2:a1
prod3:a1
catB
cat1b1
prod1:b0
prod2:b0
prod3:b0
cat2b2
prod1:b1
prod2:b1
prod3:b1
db2
catC
cat1c1
prod1:c0
prod2:c0
prod3:c0
cat2c1
prod1:c1
prod2:c1
prod3:c1
catD
cat1d2
prod1:d0
prod2:d0
prod3:d0
cat2d2
prod1:d1
prod2:d1
prod3:d1';

$array = explode("\r\n", $string);

// Or Use $trimmed = file('somefile.txt', FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);


$result = [];
foreach ($array as $item){
	
	if (mb_substr($item, 0, 2, 'UTF-8') == 'db'){
		$db = $item;
		continue;
	}
	
	$cat = mb_substr($item, -1, 1, 'UTF-8');
	
	if (ctype_upper($cat) && !is_numeric($cat)){
		$catKey = $item;
		continue;
	}
	
	
	if (is_numeric($cat) && !preg_match('/:/',$item)){
		$subCat = $item;
		continue;
	}
	
	$result[$db][$catKey][$subCat][] = $item;
}

print_r($result);

Array
(
    [db1] => Array
        (
            [catA] => Array
                (
                    [cat1a1] => Array
                        (
                            [0] => prod1:a0
                            [1] => prod2:a0
                            [2] => prod3:a0
                        )

                    [cat2a2] => Array
                        (
                            [0] => prod1:a1
                            [1] => prod2:a1
                            [2] => prod3:a1
                        )

                )

            [catB] => Array
                (
                    [cat1b1] => Array
                        (
                            [0] => prod1:b0
                            [1] => prod2:b0
                            [2] => prod3:b0
                        )

                    [cat2b2] => Array
                        (
                            [0] => prod1:b1
                            [1] => prod2:b1
                            [2] => prod3:b1
                        )

                )

        )

    [db2] => Array
        (
            [catC] => Array
                (
                    [cat1c1] => Array
                        (
                            [0] => prod1:c0
                            [1] => prod2:c0
                            [2] => prod3:c0
                        )

                    [cat2c1] => Array
                        (
                            [0] => prod1:c1
                            [1] => prod2:c1
                            [2] => prod3:c1
                        )

                )

            [catD] => Array
                (
                    [cat1d2] => Array
                        (
                            [0] => prod1:d0
                            [1] => prod2:d0
                            [2] => prod3:d0
                        )

                    [cat2d2] => Array
                        (
                            [0] => prod1:d1
                            [1] => prod2:d1
                            [2] => prod3:d1
                        )

                )

        )

)