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Using constants in PHP serves the primary purpose of defining values that cannot be changed during execution. When a constant is defined, it remains constant throughout the script, and attempting to change its value or redefine it will result in an error. This immutability ensures that certain key values—like configuration settings or important identifiers—remain consistent and dependable, which is crucial for maintaining code stability and preventing unintended side-effects from variable modifications.

For example, if you define a constant for a database connection string, you can be assured that throughout the execution of your script, that connection string will not be altered, thereby avoiding potential issues with database connections.

The other options do not accurately represent the main purpose of constants:

- Allowing variables to change dynamically contradicts the fundamental principle of constants, which is the stability of its value.

- While defining constants might have a minimal impact on execution speed due to reduced variable resolution time, improving script execution speed is not the main intent of using constants.

- Avoiding variable naming collisions is more of a function of scope management and naming conventions, rather than an inherent feature of constants themselves. While establishing constants can help in creating unique identifiers, this is a secondary benefit compared to their primary role in maintaining unchangeable values.