Only First Byte
When assigning a string to a position inside another string, PHP reports a warning: indeed, only the first byte is used.
PHP code
<?php
$str = 'xy';
// first letter is now a
$str[0] = 'a';
// second letter is now b, c is ignored
$str[1] = 'bc';
echo $str;
?>
Before
ab
After
PHP Warning: Only the first byte will be assigned to the string offset
Warning: Only the first byte will be assigned to the string offset
ab
PHP version change
This behavior changed in 8.0.