Input/output streams

The CLI SAPI defines a few constants for I/O streams to make programming for the command line a bit easier.

CLI specific Constants
Constant Description

An already opened stream to stdin. This saves opening it with

If you want to read single line from stdin, you can use
trim(fgets(STDIN)); // reads one line from STDIN
fscanf(STDIN"%d\n"$number); // reads number from STDIN


An already opened stream to stdout. This saves opening it with



An already opened stream to stderr. This saves opening it with


Given the above, you don't need to open e.g. a stream for stderr yourself but simply use the constant instead of the stream resource:

php -r 'fwrite(STDERR, "stderr\n");'
You do not need to explicitly close these streams, as they are closed automatically by PHP when your script ends.


These constants are not available if reading the PHP script from stdin.

add a note add a note

User Contributed Notes 4 notes

Aurelien Marchand
6 years ago
Please remember in multi-process applications (which are best suited under CLI), that I/O operations often will BLOCK signals from being processed.

For instance, if you have a parent waiting on fread(STDIN), it won't handle SIGCHLD, even if you defined a signal handler for it, until after the call to fread has returned.

Your solution in this case is to wait on stream_select() to find out whether reading will block. Waiting on stream_select(), critically, does NOT BLOCK signals from being processed.

ecrist at secure-computing dot net
5 years ago
The following code shows how to test for input on STDIN.  In this case, we were looking for CSV data, so we use fgetcsv to read STDIN, if it creates an array, we assume CVS input on STDIN, if no array was created, we assume there's no input from STDIN, and look, later, to an argument with a CSV file name.

Note, without the stream_set_blocking() call, fgetcsv() hangs on STDIN, awaiting input from the user, which isn't useful as we're looking for a piped file. If it isn't here already, it isn't going to be.

(STDIN, 0);
$csv_ar = fgetcsv(STDIN);
if (
"CVS on STDIN\n";
} else {
"Look to ARGV for CSV file name.\n";
ayon at hyurl dot com
2 months ago
I find a BUG with the constant STDIN, I don't know if it si the Enter/Return key that make this proprem, when I use trim(fgets(STDIN)), that doesn't trim anything, when I detect the length of fgets(STDIN), in windows, it is 2 characters longer than what I input, in Linux, it makes 1. I tried to trim(fgets(STDIN), ' \r\n'), but it still does not work.
So I have to substr the input manually, it seems like this way:
= trim(substr(fgets(STDIN), 0, (PHP_OS == 'WINNT' ? 2 : 1)));
then I get what I want really.
James Zhu
6 years ago

function ReadStdin($prompt, $valid_inputs, $default = '') {
$input) || (is_array($valid_inputs) && !in_array($input, $valid_inputs)) || ($valid_inputs == 'is_file' && !is_file($input))) {
$input = strtolower(trim(fgets(STDIN)));
$input) && !empty($default)) {
$input = $default;

// you can input <Enter> or 1, 2, 3
$choice = ReadStdin('Please choose your answer or press Enter to continue: ', array('', '1', '2', '3'));

// check input is valid file name, use /var/path for input nothing
$file = ReadStdin('Please input the file name(/var/path):', 'is_file', '/var/path');

you can add more functions if you want.
To Top