Error when using php 7

Yeah, you’re missing an entire function in your fallback logic. You haven’t defined mb_substr_split_uncode anywhere in your code.

FWIW, you probably copied a portion of your stuff (or at least, they are doing the same things as you) from here:
https://doc.wikimedia.org/mediawiki-core/master/php/Fallback_8php_source.html.

It looks like PHP 7 does not fail gracefully when using self outside of the context of a class, which shouldn’t be done anyways, and is probably a result of copy/pasta of this code from somewhere else, where it WAS included as part of a class.

I’m making this change to my code locally, but you might want to include this diff in your next release of WPT:

@@ -293,14 +293,59 @@ if ( ! function_exists( 'mb_strlen' ) ) {
 }

 if ( ! function_exists( 'mb_substr' ) ) {
+	function mb_substr_split_unicode( $str, $splitPos ) {
+		if ( $splitPos == 0 ) {
+			return 0;
+		}
+
+		$byteLen = strlen( $str );
+
+		if ( $splitPos > 0 ) {
+			if ( $splitPos > 256 ) {
+				 // Optimize large string offsets by skipping ahead N bytes.
+				 // This will cut out most of our slow time on Latin-based text,
+				 // and 1/2 to 1/3 on East European and Asian scripts.
+				$bytePos = $splitPos;
+				while ( $bytePos < $byteLen && $str[$bytePos] >= "\x80" && $str[$bytePos] < "\xc0" ) {
+					++$bytePos;
+				}
+				$charPos = mb_strlen( substr( $str, 0, $bytePos ) );
+			} else {
+				$charPos = 0;
+				$bytePos = 0;
+			}
+
+			while ( $charPos++ < $splitPos ) {
+				++$bytePos;
+				 // Move past any tail bytes
+				while ( $bytePos < $byteLen && $str[$bytePos] >= "\x80" && $str[$bytePos] < "\xc0" ) {
+					++$bytePos;
+				}
+			}
+		} else {
+			$splitPosX = $splitPos + 1;
+			$charPos = 0; // relative to end of string; we don't care about the actual char position here
+			$bytePos = $byteLen;
+			while ( $bytePos > 0 && $charPos-- >= $splitPosX ) {
+				--$bytePos;
+				// Move past any tail bytes
+				while ( $bytePos > 0 && $str[$bytePos] >= "\x80" && $str[$bytePos] < "\xc0" ) {
+					--$bytePos;
+				}
+			}
+		}
+
+		return $bytePos;
+	}
+
 	function mb_substr( $str, $start, $count = 'end' ) {
 		if ( $start != 0 ) {
-			$split = self::mb_substr_split_unicode( $str, intval( $start ) );
+			$split = mb_substr_split_unicode( $str, intval( $start ) );
 			$str   = substr( $str, $split );
 		}

 		if ( $count !== 'end' ) {
-			$split = self::mb_substr_split_unicode( $str, intval( $count ) );
+			$split = mb_substr_split_unicode( $str, intval( $count ) );
 			$str   = substr( $str, 0, $split );
 		}