If you see the warning “Warning : Use of undefined constant – assumed ‘ ‘ (this will throw an Error in a future version of PHP)” in your PHP code, it is usually an error resulting from the use of an undefined constant. This will throw an Error in a future version of PHP.
However, this error can also be caused by the presence of double-byte spaces in the code. This is because PHP recognizes a full-width space as a constant and will issue a warning if the constant is undefined.
To determine if a full-width space is the cause, review the line of code where the error occurred and check for the presence of a full-width space. Since full-width spaces are usually indistinguishable from half-width spaces, it can be difficult to identify this problem.
One way to look for full-width spaces is to paste the code in which the error occurs into a text editor and explicitly search for full-width spaces. Many text editors provide a feature for finding full-width spaces.
Also, some code editors have the ability to visually highlight full-width spaces, which can be useful.