fix string

examples/python/string.py

@@ -0,0 +1,16 @@
+import phpy
+import random
+from php import std
+
+m1 = phpy.call('memory_get_usage')
+c = 128
+total = 0
+for i in range(1, c):
+    s = phpy.call('random_bytes', 8192 * i)
+    assert s.len() == 8192 * i
+    total += s.len()
+
+m2 = phpy.call('memory_get_usage')
+print(m2 - m1)
+print(total)
+