oṃ akāro mukhaṃ sarvadharmāṇāṃ ādyanutpannatvāt āḥ hūṃ phaṭ svāhā
This mantra has a fascinating history that ties into the Arapacana alphabet. Without the magic bits it says akāro mukhaṃ sarvadharmāṇāṃ ādyanutpannatvāt.
The letter "a" (akāro) is an opening (mukhaṃ) because of the originate state of non-arising (ādyanutpannatvāt) of all dharmas (sarvadharmāṇāṃ).
In this case the letter "a" (a-kāra) reminds us of the word anutpanna 'unarisen' (an-ut-panna). And the word itself is the focus for a reflection on the nature of dharmas from a Prajñāpāramitā point of view. Because dharmas are the objects of the manas (mind sense) they are neither existent (astitā) nor non-existent (nāstitā). They arise and pass away. In denying that anything arises when we experience something, the Prajñāpāramitā is following the early Mainstream Buddhist idea that dharmas lack substance (anātman). Nothing substantial arises when we have an experience. There is an experience, but nothing real arises in the process.
The Prajñāpāramitā was written in an environment in which dharmas where considered to be really existent things (dravya). To counter this and bring the focus back to experience they used a negative rhetoric which denied the arising of dharmas entirely. In this they were also influenced by the experience of powerful meditative experiences variously called śūnyatā-vimokṣa or nirodhasamāpatti in which no dharmas arise.
The alphabet provides a whole series of words to reflect on, e.g. 44 in the Larger Prajñāpāramitā text. Having reflected on each in turn, one then reflects how they all point to the same understanding.
Later texts, particularly the Sarvatathāgata-tattvasaṃgraha, reduced this practice to just a reflection on the letter "a" and the word anutpanna. The phrase becomes a mantra in the Hevajra Tantra.