Added dependency checks for the PHP script.

src/phpsh.php

@@ -20,6 +20,16 @@
 //  this is probably not the right fix, but we need it for now
 ini_set('memory_limit', ini_get('memory_limit') * 2 . 'M');
 
+if (version_compare(PHP_VERSION, '5.0.0', '<')) {
+  fwrite(STDERR, 'Fatal error: phpsh requires PHP 5 or greater');
+  exit;
+}
+
+if ($missing = array_diff(array('pcntl','pcre','posix','tokenizer'), get_loaded_extensions())) {
+  fwrite(STDERR, 'Fatal error: phpsh requires the following extensions: ' . implode(', ', $missing));
+  exit;
+}
+
 // we buffer the output on includes so that output that gets generated by
 // includes doesn't interfere with the secret messages we pass between php and
 // python we'll capture any output and show it when we construct the shell