Arduino: String to int gets strange values

I want to convert a String to an int, and all I could find is that you have to convert the String to a char array and then cast this array to an int, but my code produces strange values and I can't figure out what the problem is.

void ledDimm(String command)
{
    // Get the Value xx from string LEDDimm=xx
    String substring = command.substring(8, command.length());

    Serial.println("SubString:");
    Serial.println(substring);
    Serial.println("SubString Length:");
    Serial.println(substring.length());

    // Create a Char Array to Store the Substring for conversion
    char valueArray[substring.length() + 1];

    Serial.println("sizeof ValueArray");
    Serial.println(sizeof(valueArray));

    // Copy the substring into the array
    substring.toCharArray(valueArray, sizeof(valueArray));

    Serial.println("valueArray:");
    Serial.println(valueArray);

    // Convert char array to an int value
    int value = int(valueArray);

    Serial.println("Integer Value:");
    Serial.println(value);

    // Write the Value to the LEDPin
    analogWrite(LEDPin, value);
}

And the serial output looks like this:

Received packet of size 11
From 192.168.1.4, port 58615
Contents:
LEDDimm=100
SubString:
100
SubString Length:
3
sizeof ValueArray
4
valueArray:
100
Integer Value:
2225

I expected to get an int with the value of 100 but the actual int is 2225?! What have I done wrong here?