Fatal Error exporting XLS

Quick solution, until it may be resolved by the plugin author in the next update:

Replace the six occurances of $str{...} with $str[...] in the function utf16_decode() after line 527 of /wp-content/plugins/advanced-cf7-db/admin/class/PHPExcel/Shared/String.php
(Tip: make a copy/backup of that file before changing it)

Or if you don’t want to pick each occurance, just replace the whole function with

	public static function utf16_decode($str, $bom_be = TRUE) {
		if( strlen($str) < 2 ) return $str;
		$c0 = ord($str[0]);
		$c1 = ord($str[1]);
		if( $c0 == 0xfe && $c1 == 0xff ) { $str = substr($str,2); }
		elseif( $c0 == 0xff && $c1 == 0xfe ) { $str = substr($str,2); $bom_be = false; }
		$len = strlen($str);
		$newstr = '';
		for($i=0;$i<$len;$i+=2) {
			if( $bom_be ) { $val = ord($str[$i])   << 4; $val += ord($str[$i+1]); }
			else {        $val = ord($str[$i+1]) << 4; $val += ord($str[$i]); }
			$newstr .= ($val == 0x228) ? "\n" : chr($val);
		}
		return $newstr;
	}

A little background, esp. for the plugin author @vsourz1td :
The “offset access syntax with curly braces” was deprecated in PHP 7.4, see https://wiki.php.net/rfc/deprecate_curly_braces_array_access

Advanced Contact form 7 DB uses the library PHPExcel to create Excel files. PHPExcel is deprecated since 2017, and should be replaced by PhpSpreadsheet.
See https://github.com/PHPOffice/PHPExcel
and https://github.com/PHPOffice/PhpSpreadsheet (“Do you need to migrate? There is an automated tool for that.”)

Edit: this topic is a duplicate of
https://www.ads-software.com/support/topic/export-to-excel-error-2/
https://www.ads-software.com/support/topic/advanced-cf7-and-php-8-0/
https://www.ads-software.com/support/topic/php8-pdf-and-excel-failed/
etc, but no answer or solution there

• This reply was modified 2 years, 6 months ago by cyrfer. Reason: duplicate topics mentioned

For me this didn’t work, I had to change way more files in PHPExcel that use curly brace indexing.

So I used PHP Rector to refactor the entire library:
composer require rector/rector –dev

modify rector.php:
use Rector\Php74\Rector\ArrayDimFetch\CurlyToSquareBracketArrayStringRector;
//register a single rule
$rectorConfig->rule(CurlyToSquareBracketArrayStringRector::class);

run:
vendor/bin/rector process .\wp-content\plugins\advanced-cf7-db\admin

[OK] 18 files have been changed by Rector