Fix appearance of search results when using server-based search

Use fixed Doxygen PHP search functions to work around Doxygen bug
https://github.com/doxygen/doxygen/issues/7892 which affected our docs
too.

Closes #19276.

docs/doxygen/regen.sh

@@ -41,6 +41,8 @@ fi
 # Still allow using incompatible version if explicitly requested.
 if [[ -z $WX_SKIP_DOXYGEN_VERSION_CHECK ]]; then
     doxygen_version=`$DOXYGEN --version`
+    # Note: remove the hack for Doxygen 1.8.19 below when changing this to a
+    # later version.
     doxygen_version_required=1.8.17
     if [[ $doxygen_version != $doxygen_version_required ]]; then
         echo "Doxygen version $doxygen_version is not supported."
@@ -114,6 +116,11 @@ esac
 #
 $DOXYGEN Doxyfile
 
+if [[ "$1" = "php" ]]; then
+    # Work around a bug in Doxygen < 1.8.19 PHP search function.
+    cp custom_search_functions.php out/html/search_functions.php
+fi
+
 if [[ "$1" = "qch" ]]; then
     # we need to add missing files to the .qhp
     cd out/html