TRENTON Gov. Chris Christie has signed legislation that calls for the phasing in of a sharp increase in the state’s unemployment tax rate, a move proponents say will spare businesses from having to immediately pay a hefty boost in the rate. The increase, needed to help restore the state’s depleted unemployment insurance fund, will now be phased in over a three-year period instead of being imposing all this year. The measure, which had received bipartisan report and was strongly backed by the business community, was signed by Christie late last month. The unemployment insurance fund went broke, in large part, because state officials diverted billions over the years to other parts of the budget, such as hospital charity care. New Jersey also has borrowed more than $1.6 billion from the federal government, one of many states that have used such funds to help cover its unemployment claims. The unemployment tax rate is set automatically by how much the fund has in reserve in March of each year. Because the fund had no reserve this year, state labor officials say employers would have been required to pay about $300 per worker during the current fiscal year, which started July 1. Instead, they will now on average pay only about $100 this year. The law seeks to protect businesses while the economy rebounds and the state seeks a permanent solution to the fund shortfall. The proposal was one of several reform measures recommended by the state’s Unemployment Insurance Task Force.