Maintaining Package Versions

Testing Version Constraints

There is a handy packagist semver checker (short for symantic versioning) that allows you to test out your version constraints.

Version Usage Documentation

Exact - You can specify the exact version of a package. This will tell Composer to install this version and this version only. If other dependencies require a different version, the solver will ultimately fail and abort any install or update procedures.
Example:

1.0.2

Range - By using comparison operators you can specify ranges of valid versions. Valid operators are >, >=, <, <=, !=.

You can define multiple ranges. Ranges separated by a space ( ) or comma (,) will be treated as a logical AND. A double pipe (||) will be treated as a logical OR. AND has higher precedence than OR.

Note: Be careful when using unbounded ranges as you might end up unexpectedly installing versions that break backwards compatibility. Consider using the caret operator instead for safety.
Examples:

>=1.0
>=1.0 <2.0
>=1.0 <1.1 || >=1.2*

Range (Hyphen) - Inclusive set of versions. Partial versions on the right include are completed with a wildcard. For example 1.0 - 2.0 is equivalent to >=1.0.0 <2.1 as the 2.0 becomes 2.0.*. On the other hand 1.0.0 - 2.1.0 is equivalent to >=1.0.0 <=2.1.0.
Example:

1.0 - 2.0

Wildcard - You can specify a pattern with a * wildcard. 1.0.* is the equivalent of >=1.0 <1.1.
Example:

1.0.*

Next Significant Release Operators

Tilde - The ~ operator is best explained by example: ~1.2 is equivalent to >=1.2 <2.0.0, while ~1.2.3 is equivalent to >=1.2.3 <1.3.0. As you can see it is mostly useful for projects respecting semantic versioning. A common usage would be to mark the minimum minor version you depend on, like ~1.2 (which allows anything up to, but not including, 2.0). Since in theory there should be no backwards compatibility breaks until 2.0, that works well. Another way of looking at it is that using ~ specifies a minimum version, but allows the last digit specified to go up.
Example:

~1.2

Caret - The ^ operator behaves very similarly but it sticks closer to semantic versioning, and will always allow non-breaking updates. For example ^1.2.3 is equivalent to >=1.2.3 <2.0.0 as none of the releases until 2.0 should break backwards compatibility. For pre-1.0 versions it also acts with safety in mind and treats ^0.3 as >=0.3.0 <0.4.0.

This is the recommended operator for maximum interoperability when writing library code.
Example:

^1.2.3