Why does PHP shell exec function work on the command prompt but not on the HTML browser?

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The PHP shell exec function runs the command on the server where the PHP script is executed. The command prompt on the server has access to system resources that may not be available on the client's browser. In addition, the HTML browser executes the PHP script on the client-side, and it doesn't have the necessary permissions to execute shell commands. Therefore, PHP shell exec function cannot be executed on the client's browser.