sold property in the Philippines and asked money to be sent here in US

Where they send their money isn't what makes the sale taxable, it is the sale itself which would be taxable or not. If your parents were residents in the US when the house was sold, it would likely need to be reported on their US tax returns. I am not familiar with the Philippine tax system to know if it was also taxable there - if it was, the US likely allows those taxes to be claimed as a reduction of US taxes otherwise owing.

If this is a large amount of potential tax, they should seriously consider hiring a competent foreign-tax accountant familiar with the US-Philippine tax treaty; likely going to cost >$1k to get the right answer [don't be tempted by a local accountant's offer to get things done for $100, which likely indicates a lack of expertise]. There may be other issues at play, such as it being a gain on a house, based on however the tax treaty between the two countries is structured [on which I have no knowledge].