We have thought of using the T4 Template Framework in Joomla 3 and have installed T4 System Plugin + T4 Blank Template on a clean Joomla 3 instalation (Joomla 3.10.11, PHP 5.6.40 and MySql 5.5.62). We have now try to make a T4 Blank Template test page, but it does not load any data other than the main components information. For example, the <HEAD> element is completely empty..
Also, we get in debug the folloing error:
Parse error: syntax error, unexpected ':', expecting ';' or '{' in ../plugins/system/t4/src/joomla3/cms/html/bootstrap.php on line 300
There you can find:
public static function alert($selector = '') :void
It seems that this problem is because T4 System Plugin its no longer supporting PHP 5.6.40 versions..
A "void" return type has been introduced in PHP 7.1
Do you have any version that still supports PHP 5.6.40?
PS. If your component no longer supports the older PHP 5.6, then make the information about it visible, so there will be no unnecessary installations and inquiries about it. :-)