Working with I/O streams in PHP

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<?php
$stdin = fopen('php://stdin', 'r');
$stdout = fopen('php://stdout', 'w');
$stderr = fopen('php://stderr', 'w');

while(!feof($stdin)){
    $line = trim(fgets($stdin));
    if(strlen($line) > 0){
        fwrite($stdout,strrev($line).PHP_EOL);
    } else {
        fwrite($stderr,"An empty line was passed".PHP_EOL);
    }
}