A strange question about external variables called by PHP functions

PHPcode $ classfield & quot; fadsdasf & quot; functionecho_blocka () {global $ classfield; echo $ classfield;} echo_blocka (); this can be normally output on a single page, but it does not output anything on the other page. it is a strange problem that PHP functions call external variables.

PHP code

  $ Classfield = "fadsdasf"; function echo_blocka () {global $ classfield; echo $ classfield;} echo_blocka ();

This can be output normally on a single page.
However, there is no output on the other page, which is depressing. Analyze the cause.

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How can I include another page? Code?
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Discussion

I used the CMS of the empire. this CMS can directly write PHP code on the page,