OK, I finally figure it our:
echo ‘[audio:my.mp3]’;
works.
but
echo “[audio: my.mp3]”;
not working.
so the reason is here:
PHP lets you define strings using single quote, double quotes or heredoc. There’s a crucial difference between the first two and for some reason many people new to PHP start using double quotes. Hopefully the following explanation will save a couple of hours debugging for the starters among us.
The most important difference between the two is that a string enclosed in double quotes is parsed by PHP. This means that any variable in it will be expanded.
echo $var; // Results in the value of $var being printed
echo ‘$var’; // Results in the word ‘$var’
echo “$var”; // Results in the value of $var being printed