My survey of several oddities and undefined behaviors in string single-byte assignment which are strictly relates to this same topic I have collected from the discussions of these days that should be addressed as a whole in some consistent way:

<?php

// Set char in string oddities (PHP 5.6.3, PHP 7.1.0-dev).
error_reporting(-1);

// Replacing a single byte.
$s = "abc";
$s[1] = "x";
echo "replace byte: ", rawurlencode($s), "\n"; // -> axc (expected)

// Set single byte with empty replacement string (undefined behavior).
// https://bugs.php.net/bug.php?id=71572
// Expected: some error.
// Actual: replaces with byte 0.
$s = "abc";
$s[1] = ""; // empty string
echo "set empty byte: ", rawurlencode($s), "\n"; // --> a%00c

// Set single byte with 2+ bytes long replacement string (undefined behavior).
// Expected: some error.
// Actual: replaces with the first byte of the replacement string.
$s = "abc";
$s[1] = "xyz";
echo "set multiple bytes: ", rawurlencode($s), "\n"; // --> axc

// Set single byte on empty string (undefined behavior).
// Expected: some error because undefined behavior (no char to replace at
// the given offset).
// Actual: type switch to array.
$s = ""; // Empty string
$s[1] = "z";
echo "set on empty string: "; var_dump($s); // -> array(1) { [1]=>string(1) "z" }

// Append to empty string.
// Expected: append "z" to empty string resulting in "z".
// Actual: type switch to array.
$s = "";
$s[] = 'z';
echo "append on empty string: "; var_dump($s); // -> array(1) { [0]=>string(1) "z" }

// Append on non-empty string.
// Expected: append "z" to "a" resulting in "az".
// Actual: fatal error.
$s = "a";
$s[] = 'z'; // -> Fatal error : [] operator not supported for strings
?>

In my opinion, the simplest thing to do is to define the "string single-byte assignment" as the replacement of the existing byte at the given offset with the first byte of the replacement string, any other case causing E_WARNING; appending $s[] = ... not allowed (there is the . operator already); unexpected conversion to array absolutely to fix!