Fix GH-8562: SplFileObject::current() returns wrong value after next()

SplFileObject::next() without READ_AHEAD cleared the cached line and
incremented current_line_num but didn't advance the stream. When called
without a preceding current() (e.g. rewind() then next()), the stream
position stayed put, so the subsequent current() read stale data.

Read a line to advance the stream when next() is called with no cached
line.

Closes GH-8562

Author: iliaal

ext/spl/spl_directory.c

@@ -2142,6 +2142,10 @@ PHP_METHOD(SplFileObject, next)
 
 	ZEND_PARSE_PARAMETERS_NONE();
 
+	if (!intern->u.file.current_line && Z_ISUNDEF(intern->u.file.current_zval)) {
+		spl_filesystem_file_read_line(ZEND_THIS, intern, true);
+	}
+
 	spl_filesystem_file_free_line(intern);
 	if (SPL_HAS_FLAG(intern->flags, SPL_FILE_OBJECT_READ_AHEAD)) {
 		spl_filesystem_file_read_line(ZEND_THIS, intern, true);

ext/spl/tests/gh8562.phpt

@@ -0,0 +1,27 @@
+--TEST--
+GH-8562 (SplFileObject::current() returns wrong result after call to next())
+--FILE--
+<?php
+$file = new SplTempFileObject();
+for ($i = 0; $i < 5; $i++) {
+    $file->fwrite("line {$i}" . PHP_EOL);
+}
+
+$file->rewind();
+$file->next();
+echo "After rewind+next: key=" . $file->key() . " current=" . trim($file->current()) . "\n";
+
+$file->rewind();
+$file->next();
+$file->next();
+echo "After rewind+next+next: key=" . $file->key() . " current=" . trim($file->current()) . "\n";
+
+$file->rewind();
+$file->current();
+$file->next();
+echo "After current+next: key=" . $file->key() . " current=" . trim($file->current()) . "\n";
+?>
+--EXPECT--
+After rewind+next: key=1 current=line 1
+After rewind+next+next: key=2 current=line 2
+After current+next: key=1 current=line 1