Solution:
Whether you can safely use PHP’s regular string search functions depends on the character encoding:
Single-byte encodings (or UTF-8)
Safe to use functions like strpos() as long as both the string being searched and the search string are in the same encoding.
In UTF-8, a byte inside a character cannot be mistaken for another character, so byte-wise functions generally work correctly.
Other multi-byte encodings
Unsafe to use regular string search functions.
Any byte within a multi-byte character might match part of another character, potentially causing false positives.
PHP string functions like strpos() work per byte, so only UTF-8 is designed to avoid this problem.
Mismatched encodings
If the string and the search string use different encodings, you must convert one to match the other.
Otherwise, searches may fail even if the characters look the same.
✅ Best Practice:
Decide on a single character encoding for your application (e.g., UTF-8).
Convert all incoming input to that encoding immediately.
Always keep your internal string processing consistent in that encoding.