Comment faire pour insérer une nouvelle valeur de la clé de la paire dans la gamme en php?

J'ai un tableau comme suit nommé $test_package_data. Pour la référence, je suis d'impression de la première de deux éléments:

Array
(
[0] => Array
(
[test_pack_id] => 9f27643023a83addd5eed41c4aade840
[test_pack_name] => Exams Combo
[test_pack_desc] => This Package contains 24 tests of Reasoning, English and Quantitative Aptitude.
Total Tests in this Package : 26
[test_pack_type_id] => 3
[test_pack_image] => 
[test_pack_validity_year] => 0
[test_pack_validity_month] => 3
[test_pack_validity_days] => 0
[test_pack_plan] => paid
[test_pack_price] => 399.00
[test_pack_no_tests] => 0
[test_pack_publish] => yes
[test_pack_sold] => 1
[test_pack_created_staff_id] => ff8d4a5ea6bf11dce105aa2fa7b959b8
[test_pack_updated_staff_id] => 81c4e3607c7e56bbf5461ef150437675
[test_pack_created_date] => 303
[test_pack_updated_date] => 1349235701
[test_pack_purchase_date] => 255
[category_detail] => Array
(
[0] => Array
(
[test_pack_category_id] => 8
[category_name] => English Language
[test_detail] => Array
(
[0] => Array
(
[test_id] => 91
[test_name] => English Language 8
)
[1] => Array
(
[test_id] => 90
[test_name] => English Language 7
)
[2] => Array
(
[test_id] => 88
[test_name] => English Language 6
)
[3] => Array
(
[test_id] => 89
[test_name] => English Language 5
)
[4] => Array
(
[test_id] => 87
[test_name] => English Language 4
)
[5] => Array
(
[test_id] => 86
[test_name] => English Language 3
)
[6] => Array
(
[test_id] => 85
[test_name] => English Language 2
)
[7] => Array
(
[test_id] => 97
[test_name] => English Language 1
)
)
)
[1] => Array
(
[test_pack_category_id] => 4
[category_name] => All Subjects
[test_detail] => Array
(
[0] => Array
(
[test_id] => 93
[test_name] => Mega 2
)
[1] => Array
(
[test_id] => 92
[test_name] => Mega 1
)
)
)
[2] => Array
(
[test_pack_category_id] => 7
[category_name] => Quantitative Aptitude
[test_detail] => Array
(
[0] => Array
(
[test_id] => 83
[test_name] => Quantitative Aptitude 8
)
[1] => Array
(
[test_id] => 82
[test_name] => Quantitative Aptitude 7
)
[2] => Array
(
[test_id] => 81
[test_name] => Quantitative Aptitude 6
)
[3] => Array
(
[test_id] => 80
[test_name] => Quantitative Aptitude 5
)
[4] => Array
(
[test_id] => 79
[test_name] => Quantitative Aptitude 4
)
[5] => Array
(
[test_id] => 78
[test_name] => Quantitative Aptitude 3
)
[6] => Array
(
[test_id] => 77
[test_name] => Quantitative Aptitude 2
)
[7] => Array
(
[test_id] => 76
[test_name] => Quantitative Aptitude 1
)
)
)
[3] => Array
(
[test_pack_category_id] => 6
[category_name] => Logical Reasoning
[test_detail] => Array
(
[0] => Array
(
[test_id] => 75
[test_name] => Reasoning 8
)
[1] => Array
(
[test_id] => 74
[test_name] => Reasoning 7
)
[2] => Array
(
[test_id] => 73
[test_name] => Reasoning 6
)
[3] => Array
(
[test_id] => 72
[test_name] => Reasoning 5
)
[4] => Array
(
[test_id] => 71
[test_name] => Reasoning 4
)
[5] => Array
(
[test_id] => 70
[test_name] => Reasoning 3
)
[6] => Array
(
[test_id] => 69
[test_name] => Reasoning 2
)
[7] => Array
(
[test_id] => 68
[test_name] => Reasoning 1
)
)
)
)
)
[1] => Array
(
[test_pack_id] => e7e95de96987cc7c89c1f0183110fb38
[test_pack_name] => Aptitude Reasoning
[test_pack_desc] => This package contains 8 tests on Reasoning.
[test_pack_type_id] => 3
[test_pack_image] => 
[test_pack_validity_year] => 0
[test_pack_validity_month] => 3
[test_pack_validity_days] => 0
[test_pack_plan] => free
[test_pack_price] => 0.00
[test_pack_no_tests] => 0
[test_pack_publish] => yes
[test_pack_sold] => 4
[test_pack_created_staff_id] => ff8d4a5ea6bf11dce105aa2fa7b959b8
[test_pack_updated_staff_id] => ff8d4a5ea6bf11dce105aa2fa7b959b8
[test_pack_created_date] => 303
[test_pack_updated_date] => 1347127387
[test_pack_purchase_date] => 110
[category_detail] => Array
(
[0] => Array
(
[test_pack_category_id] => 6
[category_name] => Logical Reasoning
[test_detail] => Array
(
[0] => Array
(
[test_id] => 75
[test_name] => Reasoning 8
)
[1] => Array
(
[test_id] => 74
[test_name] => Reasoning 7
)
[2] => Array
(
[test_id] => 73
[test_name] => Reasoning 6
)
[3] => Array
(
[test_id] => 72
[test_name] => Reasoning 5
)
[4] => Array
(
[test_id] => 71
[test_name] => Reasoning 4
)
[5] => Array
(
[test_id] => 70
[test_name] => Reasoning 3
)
[6] => Array
(
[test_id] => 69
[test_name] => Reasoning 2
)
[7] => Array
(
[test_id] => 68
[test_name] => Reasoning 1
)
)
)
)
)
)

Maintenant, je veux ajouter une nouvelle paire clé-valeur nommée [count] => '$some_value' dans ce tableau dans chaque élément de la paire clé-valeur [test_pack_purchase_date](disons après [test_pack_purchase_date] => 255. Comment dois-je faire? Quelqu'un peut-il m'aider à ce sujet? Merci à l'avance.