Use is_file over file_exists

`is_file` is faster

Author: staabm

src/Runner/Baseline/Reader.php

@@ -12,7 +12,7 @@
 use const DIRECTORY_SEPARATOR;
 use function assert;
 use function dirname;
-use function file_exists;
+use function is_file;
 use function realpath;
 use function sprintf;
 use function str_replace;
@@ -36,7 +36,7 @@ final class Reader
      */
     public function read(string $baselineFile): Baseline
     {
-        if (!file_exists($baselineFile)) {
+        if (!is_file($baselineFile)) {
             throw new CannotLoadBaselineException(
                 sprintf(
                     'Cannot read baseline %s, file does not exist',

src/Util/PHP/AbstractPhpProcess.php

@@ -15,9 +15,9 @@
 use function array_merge;
 use function assert;
 use function explode;
-use function file_exists;
 use function file_get_contents;
 use function ini_get_all;
+use function is_file;
 use function restore_error_handler;
 use function set_error_handler;
 use function trim;
@@ -140,7 +140,7 @@ public function runTestJob(string $job, Test $test, string $processResultFile):
 
         $processResult = '';
 
-        if (file_exists($processResultFile)) {
+        if (is_file($processResultFile)) {
             $processResult = file_get_contents($processResultFile);
 
             @unlink($processResultFile);