Admin check: fix MySQL UTF-8 collation check

MySQL 5.5.3 introduced the 'utf8mb4' character which allows storage of
up to 4-byte unicode characters (utf8 only allows 3 bytes) [1].

As a consequence, the current check for collation beginning with 'utf8_'
returns false if the collation is utf8mb4.

This adds a new check_api check_is_collation_utf8() function to ensure
consistent checking of collation, and removes the underscore from the
comparison string.

Fixes #20426

[1] https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html

admin/check/check_api.php

@@ -265,3 +265,12 @@ function check_print_test_warn_row( $p_description, $p_pass, $p_info = null ) {
 	$g_alternate_row = $g_alternate_row === 1 ? 2 : 1;
 	return $p_pass;
 }
+
+/**
+ * Verifies that the given collation is UTF-8
+ * @param string $p_collation
+ * @return boolean True if UTF-8
+ */
+function check_is_collation_utf8( $p_collation ) {
+	return substr( $p_collation, 0, 4 ) === 'utf8';
+}

admin/check/check_database_inc.php

@@ -364,7 +364,7 @@
 		) {
 			check_print_test_row(
 				'Table <em>' . htmlentities( $t_row['name'] ) . '</em> is using UTF-8 collation',
-				substr( $t_row['collation'], 0, 5 ) === 'utf8_',
+				check_is_collation_utf8( $t_row['collation'] ),
 				array( false => 'Table ' . htmlentities( $t_row['name'] )
 					. ' is using ' . htmlentities( $t_row['collation'] )
 					. ' collation where UTF-8 collation is required.' ) );
@@ -383,7 +383,7 @@
 					. '</em> of type <em>' . $t_row['type']
 					. '</em> on table <em>' . htmlentities( $t_table )
 					. '</em> is is using UTF-8 collation',
-					substr( $t_row['collation'], 0, 5 ) === 'utf8_',
+					check_is_collation_utf8( $t_row['collation'] ),
 					array( false => 'Text column ' . htmlentities( $t_row['field'] )
 						. ' of type ' . $t_row['type']
 						. ' on table ' . htmlentities( $t_table )