A related problem of variable function

A problem with variable function
The manual says that Echo,print is not a real function, so variable function cannot be applied. But I tried it, and it was possible.

PHP Code

  
  $t = ' echo '; $t (' 123 ');

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This one...... Is it useful? I generally direct: echo "xxx-->> $X \ n"; or Echo ("XXX"); the effect is the same.
As long as the output can be done.
It seems that usually ignore a lot of details ah, research ...
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Did the landlord run it. Can you really do that?
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$t = ' echo ';
$t (' 123 ');
Fatal error:call to undefined function echo () ...

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