php - json_encode() function adds backslash

I have a PHP Array with the word 's morgens in it. I'm using a function which escapes the single quote, json_encode() adds another backslash to it which causes an error in the SQL code. The parameter JSON_UNESCAPED_SLASHES causes the following error:

Warning: json_encode() expects parameter 2 to be long, string given

The output is as follows, before json_encode():

string(11) "\'s middags"

And after json_encode():

"\\'s middags"

Its equivalent (long, 64) won't work, it doesn't throw an error. How can I prevent json_encode() from adding a backslash?

(Sorry for bad English, it's not my native language)