What is the MySQL query equivalent of PHP strip_tags?

I don't believe there's any efficient way to do this in MySQL alone.

MySQL does have a REPLACE() function, but it can only replace constant strings, not patterns. You could possibly write a MySQL stored function to search for and replace tags, but at that point you're probably better off writing a PHP script to do the job. It might not be quite as fast, but it will probably be faster to write.

MySQL >= 5.5 provides XML functions to solve your issue:

SELECT ExtractValue(field, '//text()') FROM table;

Reference: https://dev.mysql.com/doc/refman/5.5/en/xml-functions.html

Here you go:

CREATE FUNCTION `strip_tags`($str text) RETURNS text
BEGIN
    DECLARE $start, $end INT DEFAULT 1;
    LOOP
        SET $start = LOCATE("<", $str, $start);
        IF (!$start) THEN RETURN $str; END IF;
        SET $end = LOCATE(">", $str, $start);
        IF (!$end) THEN SET $end = $start; END IF;
        SET $str = INSERT($str, $start, $end - $start + 1, "");
    END LOOP;
END;

I made sure it removes mismatched opening brackets because they're dangerous, though it ignores any unpaired closing brackets because they're harmless.

mysql> select strip_tags('<span>hel<b>lo <a href="world">wo<>rld</a> <<x>again<.');
+----------------------------------------------------------------------+
| strip_tags('<span>hel<b>lo <a href="world">wo<>rld</a> <<x>again<.') |
+----------------------------------------------------------------------+
| hello world again.                                                   |
+----------------------------------------------------------------------+
1 row in set

I am passing this code on, seems very similar to the above. Worked for me, hope it helps.

BEGIN
  DECLARE iStart, iEnd, iLength   INT;

  WHILE locate('<', Dirty) > 0 AND locate('>', Dirty, locate('<', Dirty)) > 0
  DO
    BEGIN
      SET iStart = locate('<', Dirty), iEnd = locate('>', Dirty, locate('<', Dirty));
      SET iLength = (iEnd - iStart) + 1;
      IF iLength > 0 THEN
        BEGIN
          SET Dirty = insert(Dirty, iStart, iLength, '');
        END;
      END IF;
    END;
  END WHILE;
  RETURN Dirty;
END