You can use the following PHP code to search for a file named "filename.txt" in all directories using the shell_exec() function:
<?php $command = "find / -name 'filename.txt' 2>&1"; $output = shell_exec($command); echo "<pre>$output</pre>"; ?>
This code uses the "find" command to search for the file named "filename.txt" in the root directory and all subdirectories. The "2>&1" at the end of the command redirects any error messages to the standard output, which allows them to be captured by the shell_exec() function.
The output of the command is captured in the $output variable and displayed using the <pre> tag, which formats the output as preformatted text.
Note that this code may take some time to execute, depending on the size of your file system and the number of files it contains. It may also require elevated permissions to search directories that your user account does not have access to.