ESV Study Bible : Phil 3:9

and be found in him, not having a righteousness of my own that comes from the law, but that which comes through faith in Christ, the righteousness from God that depends on faith—Phil 3:9

Phil. 3:9 Found in him means being spiritually united to Christ and therefore found not guilty before God as divine judge. Paul had trusted in a righteousness of my own based on obedience to the law rather than the right standing before God that comes through faith in Christ. God “imputes” Christ's lifelong record of perfect obedience to the person who trusts in him for salvation; that is, he thinks of Christ's obedience as belonging to that person, and therefore that person stands before God not as “guilty” but as “righteous.” This is the basis on which justification by faith alone is considered “fair” in God's sight. As explained in Rom. 10:1–8, righteousness cannot come by the law because all human beings sin, and therefore right standing before God as the divine judge is possible only through faith in Jesus Christ, who is the believer's righteousness before God. See note on Gal. 2:16. « Less