1.2.884. Only First Byte Will Be Assigned

When assigning a char to a string with an array notation, only the first byte is used.

<?php
    $str = 'xy';

    // first letter is now a
    $str[0] = 'a';

    // second letter is now b, c is ignored
    $str[1] = 'bc';
?>

See also String access and modification by character.