如何阅读fopen（）错误

It seems that whatever I try, I don't get data in a fifo file.

$fifo_action = fopen( "/tmp/fifo_action", "w+" );
    die( "first fifo fail" ); 
$fifo_value = fopen( "/tmp/fifo_value", "w+" );
    die( "second fifo fail" );

fwrite( $fifo_value, $id );
fwrite( $fifo_action, "ly" );

fclose( $fifo_value );
fclose( $fifo_action );

Whatever I try, I get "first fifo fail" as result. The fifo's exists. I tried to chmod them in 777. I can write to the fifo from c++. Even if I login as the www-data user in my terminal.

At this time, I have no clue on how to proceed further. Is there a way to output the exact error?

You are running the die() regardless of what happens in the first fopen() so it is bound to tell you it has failed.

fopen() will return false if it fails so you have to test for that. Note the use of === which is also important when a function can return a good status that might also be interpreted accidentally as false.

$fifo_action = fopen( "/tmp/fifo_action", "w+" );
if ( $fifo_action === false){
    die( "first fifo fail" ); 
}
$fifo_value = fopen( "/tmp/fifo_value", "w+" );
if ( $fifo_value === false){
    die( "second fifo fail" );
}

fwrite( $fifo_value, $id );
fwrite( $fifo_action, "ly" );

fclose( $fifo_value );
fclose( $fifo_action );

Your die statement is executed regardless the result of fopen.

$fifo_action = fopen( "/tmp/fifo_action", "w+" )
if (!$fifo_action) die( "first fifo fail" ); 
$fifo_value = fopen( "/tmp/fifo_value", "w+" )
if (!$fifo_value) die( "second fifo fail" );

fwrite( $fifo_value, $id );
fwrite( $fifo_action, "ly" );

fclose( $fifo_value );
fclose( $fifo_action );