MultibytePositionUsedAsByteOffset

warning native on by default

Group: Probable bugs

Multibyte character positions are not byte offsets. Use matching multibyte operations when slicing at a multibyte search result.

Example

<?php
$s="été";$p=mb_strpos($s,"t",0,"UTF-8");echo substr($s,$p,1);

Reported:

• line 2: Use multibyte offsets with multibyte slicing.

Detection

• Resolved mb_strpos/mb_stripos on a known UTF-8 literal containing multibyte characters before a found needle yields index used as offset of substr on same source. Require literal needle and explicit UTF-8 encoding, no arithmetic conversion.
• Resolve builtin functions, classes, constants and methods; user-defined lookalikes are excluded. Follow only bounded local proof in one lexical scope.
• Emit one finding per violating operation, not per supporting evidence statement. When the proof is inconclusive, nothing is reported.

Exceptions (no report)

• Do not report unresolved or shadowed builtin names, unknown values, or incomplete receiver/type provenance. Unknown calls, aliases escaping, and intervening writes invalidate local proof. Similar code outside the stated detection contract is not a finding.
• Both standard suppression forms suppress this native ID.

Fix

None. Choosing a repair requires intent or additional runtime information.

PHP versions

Available throughout supported PHP 5.3–8.5 where the referenced builtin/extension is available. Respect syntax/version-specific receiver contracts; unavailable APIs are not this rule’s concern.

Configure

In custos.json:

{
  "rules": {
    "MultibytePositionUsedAsByteOffset": {
      "enabled": false
    }
  }
}

Suppress

Before the statement or declaration (or the first statement of the file):

// @custos-ignore MultibytePositionUsedAsByteOffset