<?php
$string = 'Date, Time, ABKtMSUNT000 [h], ABKtMWDIR012 [Deg.M], ABKtMWSPD012 [m/s],
ABKtMTEMP002 [deg.C], ABKtMRAIN000 [mm/h] 190227, 2020, 9, 245.8, 2.886, 5.753, 0';
$pr = preg_replace(['/(?<!,)(?=\h\d+)/', '/\w+\h\[([^]]+)\]/'], [',', '$1'], $string);
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