diff --git a/exts/jphp-zend-ext/src/main/tests/org/develnext/jphp/zend/ext/standard/StringFunctionsTest.java b/exts/jphp-zend-ext/src/main/tests/org/develnext/jphp/zend/ext/standard/StringFunctionsTest.java
new file mode 100644
index 000000000..466621590
--- /dev/null
+++ b/exts/jphp-zend-ext/src/main/tests/org/develnext/jphp/zend/ext/standard/StringFunctionsTest.java
@@ -0,0 +1,99 @@
+package org.develnext.jphp.zend.ext.standard;
+
+import org.develnext.jphp.zend.ext.standard.StringFunctions;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class StringFunctionsTest {
+
+  @Test
+  public void testToUUChar() {
+    Assert.assertEquals('`', StringFunctions.toUUChar(0));
+    Assert.assertEquals(' ', StringFunctions.toUUChar(64));
+  }
+
+  @Test
+  public void testIsWhitespace() {
+    Assert.assertFalse(StringFunctions.isWhitespace('\u0000'));
+    Assert.assertTrue(StringFunctions.isWhitespace('\t'));
+    Assert.assertTrue(StringFunctions.isWhitespace('\r'));
+    Assert.assertTrue(StringFunctions.isWhitespace('\n'));
+    Assert.assertTrue(StringFunctions.isWhitespace(' '));
+  }
+
+  @Test
+  public void testToUpperCase() {
+    Assert.assertEquals('\u0000', StringFunctions.toUpperCase('\u0000'));
+    Assert.assertEquals('{', StringFunctions.toUpperCase('{'));
+    Assert.assertEquals('Z', StringFunctions.toUpperCase('z'));
+  }
+
+  @Test
+  public void testToHexChar() {
+    Assert.assertEquals('c', StringFunctions.toHexChar(12));
+    Assert.assertEquals('0', StringFunctions.toHexChar(0));
+  }
+
+  @Test
+  public void testToUpperHexChar() {
+    Assert.assertEquals('8', StringFunctions.toUpperHexChar(8));
+    Assert.assertEquals('A', StringFunctions.toUpperHexChar(10));
+  }
+
+  @Test
+  public void testHexToDigit() {
+    Assert.assertEquals(0, StringFunctions.hexToDigit('0'));
+    Assert.assertEquals(15, StringFunctions.hexToDigit('F'));
+    Assert.assertEquals(15, StringFunctions.hexToDigit('f'));
+    Assert.assertEquals(-1, StringFunctions.hexToDigit('`'));
+    Assert.assertEquals(-1, StringFunctions.hexToDigit('@'));
+    Assert.assertEquals(-1, StringFunctions.hexToDigit(' '));
+    Assert.assertEquals(-1, StringFunctions.hexToDigit('r'));
+  }
+
+  @Test
+  public void testOctToDigit() {
+    Assert.assertEquals(0, StringFunctions.octToDigit('0'));
+    Assert.assertEquals(-1, StringFunctions.octToDigit('8'));
+    Assert.assertEquals(-1, StringFunctions.octToDigit(' '));
+  }
+
+  @Test
+  public void testLtrim() {
+    Assert.assertEquals("1a 2b 3c", StringFunctions.ltrim(" 1a 2b 3c"));
+    Assert.assertEquals("3", StringFunctions.ltrim("3", "2"));
+    Assert.assertEquals("", StringFunctions.ltrim("3", "A1B2C3"));
+  }
+
+  @Test
+  public void testRtrim() {
+    Assert.assertEquals("", StringFunctions.rtrim("1"));
+    Assert.assertEquals("1234", StringFunctions.rtrim("1234"));
+    Assert.assertEquals("", StringFunctions.rtrim("\'", "2"));
+    Assert.assertEquals("1234", StringFunctions.rtrim("1234", "2"));
+    Assert.assertEquals("a\'b\'", StringFunctions.rtrim("a\'b\'c", "1a 2b 3c"));
+  }
+
+  @Test
+  public void testStrcoll() {
+    Assert.assertEquals(-1, StringFunctions.strcoll(",", "bar"));
+    Assert.assertEquals(1, StringFunctions.strcoll("foo", "bar")); 
+    Assert.assertEquals(0, StringFunctions.strcoll("foo", "foo"));
+  }
+
+  @Test
+  public void test_substr_replace() {
+    Assert.assertEquals("barfoobar",
+        StringFunctions._substr_replace("foobar", "bar", 0, 0));
+    Assert.assertEquals("foobarbar",
+        StringFunctions._substr_replace("foobar", "bar", 7, 0));
+    Assert.assertEquals("foobabarr",
+        StringFunctions._substr_replace("foobar", "bar", -1, 0));
+    Assert.assertEquals("bar",
+        StringFunctions._substr_replace("", "bar", -1, 0));
+    Assert.assertEquals("barr",
+        StringFunctions._substr_replace("foobar", "bar", 0, -1));
+    Assert.assertEquals("bar",
+        StringFunctions._substr_replace("foobar", "bar", 0, 7));
+  }
+}