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 );
?>