I'm not sure I understand. Doesn't a $length imply that strpbrk will only check the first $length characters of $haystack? How does that help your use case of checking "if a single character, among a list of characters, exists in a string"?

Instead, have you seen strcspn? https://3v4l.org/G3orE

My objective was to avoid introducing a new core function by enhancing an existing function in a BC way. strpbrk() seemed to fit the bill neatly. The description of $length was borrowed from substr() which, if positive, indicates the maximum length of the returned string. If strpbrk() is enhanced as recommended, then I would implement as follows:

if( strlen( strpbrk( $string, $allowed, 1 ) ) ){ /* .. do something */}

Here is an example of a userland implementation (https://3v4l.org/cp19M) but the obvious benefit of implementing at the core level is to avoid returning the remaining segment of the string once a character is found.

Thank you for pointing out strcspn(). Based on your example, the inline implementation is similar, albeit longer, to the strpbrk() variant shown above:

if( strcspn( $string, $allowed ) < strlen( $string ){ /* .. do something */}

To cover edge cases, I wrapped it up and added it to my library (https://3v4l.org/0C6sQ).

Thanks.

This needs discussion, for which this bug tracker is not suitable.
Feel free to pursue the RFC process[1].  For the time being, I'm
suspending this ticket.

[1] <https://wiki.php.net/rfc/howto>