To text a heart symbol on a cell phone, press the "<" button, and then the "3" button, effectively making a heart that looks like this: <3.
Another way to text a heart symbol is through an emoji. On an iPhone, the emoji keyboard comes preinstalled and includes many different types of heart emojis that can easily be texted. Android phones as well have pre-installed emoji keyboards that include heart emojis that can easily be texted.
A heart, being a symbol of love, is a popular emoji to send as it is aesthetically pleasing to the eye as well as being symmetric.