How to convert strings to symbolic expressions?

Question

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Here is a function I defined:

function [X]=Taylor(a,b,h,p,m)
n=(b-a)/h;
X=zeros(1,n+1);
X(1)=p;
A=sym(zeros(1,m));
syms t;
f=str2sym('t+x(t)^2');
A(1)=f;
df=f;
for i=2:m
    df0=diff(df,t);
    df=subs(df0,'diff(x(t),t)','t+x(t)^2');
    A(i)=df;
end
A=subs(A,'x(t)','y');
for i=1:n
    B=A;
    B=subs(B,'y',X(i));
    B=subs(B,'t',a+(i-1)*h);
    C=zeros(1,m+1);
    for k=1:m
        C(k+1)=B(k)*h^k/factorial(k);
        C(1)=C(1)+C(k+1);
    end
    X(i+1)=X(i)+C(1);
end
end

When I tried to use this function, however, I got an error message. Here is the error message:

>> [X]=Taylor(a,b,h,p,m)
Error using sym>convertChar (line 1537)
Character vectors and strings in the first argument can only specify a variable or number. To evaluate character vectors and strings representing symbolic expressions, use 'str2sym'.
Error in sym>tomupad (line 1253)
        S = convertChar(x);
Error in sym (line 220)
                S.s = tomupad(x);
Error in sym/subs>inputchk (line 249)
      (length(sym(x))~=1 || length(sym(y))~=1) && isvarname(char(x))
Error in sym/subs>mupadsubs (line 154)
error(inputchk(X,Y));
Error in sym/subs (line 145)
    G = mupadsubs(F,X,Y);
Error in Taylor (line 17)
    df=subs(df0,'diff(x(t),t)','t+x(t)^2');

In line 17, I tried everything to fix this error, but none of them works. I guess I should convert my string expression to symbolic expression somewhere, but I don't know how to do. Please, can anyone help me? Thank you so much!