Update solution-php.md

Author: tanhongit

medium/840. Magic Squares In Grid/solution-php.md

@@ -25,13 +25,9 @@ To determine how many `3x3` subgrids within a given grid are magic squares, we n
     If the helper function returns true, increment the counter `$res`.
 
 # Complexity
-- **Time complexity**:
+- **Time complexity**: **$O(\text{m} \times \text{n})$** is the number of rows and (n) is the number of columns in the grid. We iterate over each possible `3x3` subgrid and check if it is a magic square.
 
-`O(m×n)` is the number of rows and (n) is the number of columns in the grid. We iterate over each possible `3x3` subgrid and check if it is a magic square.
-
-- **Space complexity**:
-
-`O(1)`, aside from the input grid, as we use a constant amount of extra space to store the set of numbers and the sums.
+- **Space complexity**: $O(1)$, aside from the input grid, as we use a constant amount of extra space to store the set of numbers and the sums.
 
 # Code
 ```php