Php shell script execution problems

Php executes shell scripts on the xamp environment platform on ubuntu. When using php shell_exec to execute shell scripts, only some commands can be executed, or only echo commands can be executed, such as cat & nbsp; and more, and no response information is returned, for example: echo & nbsp; "a" php shell script execution problem
Xamp environment platform built on ubuntu. When using php shell_exec to execute shell scripts, only some commands can be executed, or only echo commands can be executed, such as cat and more, without any response information, such:
Echo ""
Cat a.txt
More B .txt
Echo "B"
The result is: AB. the two commands in the middle are not executed, and no feedback is provided. why?
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