Instructions
Begin by downloading Cheat Engine, which will allow you to get around the game's parameters. Visit CheatEngine.org, click on "downloads," select the most current Cheat Engine version and install.
Start a new game of "Plants vs. Zombies" once Cheat Engine is installed. Check to see how much sun you have and write down the number (probably 50). Keep this information nearby as you will need it shortly.
Run Cheat Engine by holding down the "Alt" button and push "Tab." From the program screen select "Process List" and then "Windows List" from the pop-up window. From the list, pick "Plants vs. Zombies" and enter the amount of sun you had when you started the game in the "value" field. Select "First Scan" or "New Scan," depending on which version of Cheat Engine you are running. Once the scan is complete, return to the game, play for about a minute and write down your number of suns. Return to the Cheat Engine window and, as you did before, enter the number of sun you currently have in the "Value" field and select "First Scan" or "New Scan." Under "Address" and "Value," you will see a line of code. Click on it. When the same line appears at the bottom of the window, click "Value" and enter 99999999 in the field. Close Cheat Engine and save the changes. You now have unlimited sun!
Open Cheat Engine again by holding down "Alt" and pressing "Tab." Click on the computer icon in the top corner. In the pop-up window, scroll down and choose "popcapgame1.exe." Click on "Memory View" and then right-click on any number. Select "Go to Address" and type "0048728C" in the box. Double-Click on the highlighted number. Change it from "add dword ptr [edi+24],01" to "add dword ptr [edi+24],200." Press "OK" and exit Cheat Engine, saving when prompted. You now have instant seed refills on "Plants vs. Zombies"!