The code I posted above is for when you use a ADS1115 16-bit ADC. If you’re using a standard Uno or Mega board, you’ll have to use the regular 10-bit ADC, unless you’re planning to use a separate ADS1115 breakout. But assuming you’re okay with the lower resolution of the 10-bit ADC, just use the standard analogRead command and then convert the result to voltage. And because the output of the SP-212 is 1mv = 1 PAR, the measured voltage (in millivolts) is the PAR. The 10-bit resolution of the ADC means you’ll get steps of around 5 PAR (4.89 actually).
int sensorValue = analogRead(A0); //change this to the correct analog pin you're using
float voltage = sensorValue * (5.0 / 1023.0); //converts bits to volts
float PAR = voltage * 1000.0; //converts volts to PAR (for SP-212, 1 mV = 1 PAR)
Serial.print("Bits: "); Serial.print(sensorValue);
Serial.print(" PAR: "); Serial.println(PAR);