Be more lax in function/methodnames tag in regards to whitespace

[Girgias]

I recently fixed an issue on pt_BR where linking on <function> was failing because whitespace inside the tag. So I suggest trim() and strtolower() the tag's content before calculating link targets.

Originally posted by @alfsb in php/doc-en#2658 (comment)

[alfsb]

For example, the first $_POST does not link, because <varname> is broken in two lines on XML sources, as now.

Source: https://github.com/php/doc-en/blob/be90f2a70062d28df2bfe951f0fa97faee06005f/faq/using.xml#L45

      predefined variables</link>, like the superglobal <varname>
      $_POST</varname>.  You may loop through <varname>$_POST</varname>

Render:

[alfsb]

A similar situation occurs with <constant>:

Source was: php/doc-en@0b26a7a#diff-717ba49111eccd44cb75e6b39de78d12c36f2d625be55b911055f5a5be133081L733