MDL-67030 htmlpurifier: php74 fixup for htmlpurifier v4.10.0

This simply applies this pull request:

https://github.com/ezyang/htmlpurifier/pull/224

straight to our library. Once there is some new htmlpurifier
release available (>v4.11.0) we'll be back upstream because
the fix will be included.

lib/htmlpurifier/HTMLPurifier/ChildDef/Custom.php

@@ -45,7 +45,7 @@ class HTMLPurifier_ChildDef_Custom extends HTMLPurifier_ChildDef
     protected function _compileRegex()
     {
         $raw = str_replace(' ', '', $this->dtd_regex);
-        if ($raw{0} != '(') {
+        if ($raw[0] != '(') {
             $raw = "($raw)";
         }
         $el = '[#a-zA-Z0-9_.-]+';

lib/htmlpurifier/HTMLPurifier/Encoder.php

@@ -159,7 +159,7 @@ class HTMLPurifier_Encoder
 
         $len = strlen($str);
         for ($i = 0; $i < $len; $i++) {
-            $in = ord($str{$i});
+            $in = ord($str[$i]);
             $char .= $str[$i]; // append byte to char
             if (0 == $mState) {
                 // When mState is zero we expect either a US-ASCII character

lib/htmlpurifier/HTMLPurifier/TagTransform/Font.php

@@ -75,7 +75,7 @@ class HTMLPurifier_TagTransform_Font extends HTMLPurifier_TagTransform
         if (isset($attr['size'])) {
             // normalize large numbers
             if ($attr['size'] !== '') {
-                if ($attr['size']{0} == '+' || $attr['size']{0} == '-') {
+                if ($attr['size'][0] == '+' || $attr['size'][0] == '-') {
                     $size = (int)$attr['size'];
                     if ($size < -2) {
                         $attr['size'] = '-2';