Hola @Luminis :)
Este código puede servirte:    
Código PHP:
Ver original- $str1 = 'gained 1 point'; 
- $number1 = implode(' ', $matches[0]); 
-   
- $str2 = 'gained 100 points'; 
- $number2 = implode(' ', $matches[0]); 
-   
- $str3 = 'gained 1000 points'; 
- $number3 = implode(' ', $matches[0]); 
-   
- echo $number1 + $number2 + $number3; 
Saludos!