PHP standard input?

It is possible to read the stdin by creating a file handle to php://stdin and then read from it with fgets() for a line for example (or, as you already stated, fgetc() for a single character):

<?php
$f = fopen( 'php://stdin', 'r' );

while( $line = fgets( $f ) ) {
  echo $line;
}

fclose( $f );
?>

Reading from STDIN is recommended way

<?php
while (FALSE !== ($line = fgets(STDIN))) {
   echo $line;
}
?>

To avoid having to mess around with filehandles, use file_get_contents() and php://stdin:

$ echo 'Hello, World!' | php -r 'echo file_get_contents("php://stdin");'
Hello, World!

(If you're reading a truly huge amount of data from stdin you might want to use the filehandle approach, but this should be good for many megabytes.)