By the way you know how i can resolve this please

<xsl:for-each select="../../SELECT/$choice">

This is the same as expecting your PHP to produce 5 as result of
 $ex=" + 3";
 $result= 2 . $ex;
The resolution of the problem depends on what exactly gets into
$choice, if it's a single element name, try
 <xsl:for-each select="../../SELECT/*[name()=$choice]">

Check the FAQ (you've got a link already) for more on this topic,
search for "evaluate".

Note further that the way you've defined the variable $choice means
it holds a result tree fragment (RTF), which may trap you elswhere
too. Again, check the FAQ.