Can’t Change Theme And White Screen When Activating Any Plugin

As described in this SO answer – https://stackoverflow.com/questions/8028957/how-to-fix-headers-already-sent-error-in-php whitespace before the opening <?php tag of a file can cause a “headers already sent error”.

In particular, if the error message is indicating the problem is on line one of the file, this can point to leading whitespace before the first <?php tag (mentioned in point 3 of the above post).

Removing this whitespace from the file should correct the problem.

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