Error making an Angular POST request (Syntaxerror: Unexpected token s in JSON at position 0 at JSON.parse)

Question

Angular:

public urlConvenio: string = "http://localhost/teste/sistema/backend/controller/convenio/listar.php";

const httpOptions = {
   headers: new HttpHeaders({
     'Content-Type': 'application/json'
   })
};

//REQUISIÇÃO
public enviaDados() {
    this.http.post(this.urlPost, 
                   JSON.stringify(this.myForm.value), 
                   this.httpOptions).subscribe(res => {
       console.log(res);
    })
 }

Error:

Server:

Answer 1

Add the header responseType in his constant httpOptions

 const httpOptions = {
        headers: new HttpHeaders({ 'Content-Type': 'application/json', 'responseType': 'text' })
    }