strpos

strpos is a string-searching function in PHP. It returns the position of the first occurrence of a substring (the needle) within another string (the haystack). The search can start at an optional offset, and the return value is the index at which the needle occurs, or false if the needle is not found. The index is 0-based.

If the needle is an empty string, strpos returns 0. When using the result in conditional logic, it is important to compare strictly to false, because a valid match at position 0 evaluates to false in a loose comparison.

There are related functions: stripos performs a case-insensitive search; strrpos returns the last occurrence; mb_strpos is available for multibyte encodings. The third parameter defines the offset from which to start the search; a negative offset is counted from the end of the haystack; if the offset points beyond the string, the function returns false.

Notes: strpos is designed for simple substring searching and is not a regular expression engine; for patterns use preg_match. It is binary-safe, meaning it can search in binary data and does not assume character encoding.