Error: Cannot modify header information

[phpBB Debug] PHP Notice: in file / includes / functions.php on line 3545: Can not modify header information – headers already sent by (output started at / index.php: 1)
Reasons:

Your editor has added at the beginning of the file that is listed at the end of the communication (in the above example, it is the file index.php) the BOM.
RESOLUTION

Change editor to one that supports UTF8 encoding without BOM. The list of the relevant editors can be found on our website editors in the category code.
Alternatively,

If the removal of trade BOM did not help, to check whether at the beginning of the file before
PHP
or at the end, after

there is no space, or enter some other character – if it is to remove it.