Get first string before separator?

Question

I have strings with folowing structure:

7_string_12
7_string2_122
7_string3_1223

How I can get string before second "_" ?

I want my final result to be :

7_string
7_string2
7_string3

I am using explode('_', $string) and combine first two values, but my script was very slow!

Answer 1

$str = '7_string_12';
echo substr($str,0,strrpos($str,'_'));

echoes

7_string

no matter what's at the begining of the string