diff --git a/phpBB/includes/functions_messenger.php b/phpBB/includes/functions_messenger.php
index 52c205ceb5..36ce5344a5 100644
--- a/phpBB/includes/functions_messenger.php
+++ b/phpBB/includes/functions_messenger.php
@@ -1847,7 +1847,7 @@ class smtp_class
  * https://github.com/symfony/polyfill-iconv/blob/fd324208ec59a39ebe776e6e9ec5540ad4f40aaa/Iconv.php#L355
  *
  * @param string $str
- * @param string $eol End of line we are using (optional to be backwards compatible)
+ * @param string $eol Lines delimiter (optional to be backwards compatible)
  *
  * @return string
  */
@@ -1856,20 +1856,20 @@ function mail_encode($str, $eol = "\r\n")
 	// Check if string contains ASCII only characters
 	$is_ascii = strlen($str) === utf8_strlen($str);
 
-	$scheme = $is_ascii ? "Q" : "B";
+	$scheme = $is_ascii ? 'Q' : 'B';
 
 	// Define start delimimter, end delimiter
 	// Use the Quoted-Printable encoding for ASCII strings to avoid unnecessary encoding in Base64
-	$start = $is_ascii ? "=?US-ASCII?$scheme?" : "=?UTF-8?$scheme?";
-	$end = "?=";
+	$start = $is_ascii ? '=?US-ASCII?' . $scheme . '?' : '=?UTF-8?' . $scheme . '?';
+	$end = '?=';
 
 	// Maximum encoded-word length is 75 as per RFC 2047 section 2.
-	// $split_length *must* be a multiple of 4, but <= 75 - strlen($start . $delimiter . $end)!!!
+	// $split_length *must* be a multiple of 4, but <= 75 - strlen($start . $eol . $end)!!!
 	$split_length = 75 - strlen($start . $eol . $end);
 	$split_length = $split_length - $split_length % 4;
 
-	$line_length = \strlen($start) + \strlen($end);
-	$line_offset = \strlen($start) + 1;
+	$line_length = strlen($start) + strlen($end);
+	$line_offset = strlen($start) + 1;
 	$line_data = '';
 
 	$is_quoted_printable = 'Q' === $scheme;
@@ -1885,7 +1885,7 @@ function mail_encode($str, $eol = "\r\n")
 				'/[=_\?\x20\x00-\x1F\x80-\xFF]/',
 				function ($matches)
 				{
-					$hex = dechex(\ord($matches[0]));
+					$hex = dechex(ord($matches[0]));
 					$hex = strlen($hex) == 1 ? "0$hex" : $hex;
 					return '=' . strtoupper($hex);
 				},
@@ -1905,7 +1905,7 @@ function mail_encode($str, $eol = "\r\n")
 		}
 
 		$line_data .= $char;
-		$is_quoted_printable && $line_length += \strlen($char);
+		$is_quoted_printable && $line_length += strlen($char);
 	}
 
 	if ('' !== $line_data)
diff --git a/tests/email/headers_encoding_test.php b/tests/email/headers_encoding_test.php
new file mode 100644
index 0000000000..a884a4cc21
--- /dev/null
+++ b/tests/email/headers_encoding_test.php
@@ -0,0 +1,48 @@
+<?php
+/**
+ *
+ * This file is part of the phpBB Forum Software package.
+ *
+ * @copyright (c) phpBB Limited <https://www.phpbb.com>
+ * @license GNU General Public License, version 2 (GPL-2.0)
+ *
+ * For full copyright and license information, please see
+ * the docs/CREDITS.txt file.
+ *
+ */
+
+class phpbb_headers_encoding_test extends phpbb_test_case
+{
+	protected function setUp(): void
+	{
+		global $phpbb_root_path, $phpEx;
+
+		if (!function_exists('mail_encode'))
+		{
+			include($phpbb_root_path . 'includes/functions_messenger.' . $phpEx);
+		}
+	}
+
+	public function headers_encoding_data()
+	{
+		return [
+			['test@yourdomain.com <phpBB fake test email>', 'Q', 'US-ASCII'],
+			['test@yourdomain.com <Несуществующий почтовый адрес phpBB>', 'B', 'UTF-8'],
+			["\xE3\x83\x86\xE3\x82\xB9\xE3\x83\x88\xE3\x83\x86\xE3\x82\xB9\xE3\x83\x88", 'B', 'UTF-8'],
+		];
+	}
+
+	/**
+	 * @dataProvider headers_encoding_data
+	 */
+	public function test_headers_encoding($header, $scheme, $encoding)
+	{
+		$encoded_string = mail_encode($header);
+		$this->assertStringStartsWith("=?$encoding?$scheme?", $encoded_string);
+		$this->assertStringEndsWith('?=', $encoded_string);
+
+		// Result of iconv_mime_decode() on decoded header should be equal to initial header
+		$decoded_string = iconv_mime_decode($encoded_string, 0, $encoding);
+		$this->assertEquals(0, strcmp($header, $decoded_string));
+	}
+}