How does PHP use const to define another constant with one constant?

ConstX & #039; x & #039; constXXX. & #039; x & #039; the second line has a syntax error. What is the correct format. As shown below

Const X = 'X ';
Const XX = X. 'X ';

The second line has a syntax error. What is the correct format.

Reply content:

As shown below

Const X = 'X ';
Const XX = X. 'X ';

The second line has a syntax error. What is the correct format.

Upgrade PHP5.6

Http://cn2.php.net/manual/zh/migration56.new-features.php#migration56.new-features.const-scalar-exprs

Constant Value; only scalar and null are allowed. The scalar type is integer, float, string, or boolean.
It can also define the type of constant value as resource, but it is not recommended to do so, it may cause unknown conditions.

Solution:

Const X1 = 'x1'; const X2 = 'x2'; echo X1.X2;

Of course you can also choose to upgrade, as @ mcfog said ~