iirc, php 5.4 is the minimum required for ubbt 7.7.5
ubbt 7.5 requires at least php 5.6 to be installed, with php 7.1 and newer as the recommendation.
once you have php 5.6 or php 7.1 to 7.4 installed, you should not expect any issues with your upgrade to ubbt 7.7.5.
php5.2 has time zone (clock) issues, and does not support all of the php commands required, such as conventional array string usage. thats just two of the many bugs with in php 5.2 language version.